Improve LRC output by removing SRT styles

[v2lmmj04]

This addresses #20 .

ASS files, or even SRT files, may contain styles / markup that are not desirable to exist as part of the final output when exporting subtitles (especially LRC files).

This updates the ffmpeg command when converting subtitles to use a plain text format that removes any sort of styling. It results in SRT files also always being "converted" as a result, as they may have styling that needs removed as well.

It may not handle cases where convert_sub_if_needed is not called, such as through the condense_multi -> extract_srt code path that seems to skip calling this function.

I functionally tested by running .\make_exe.bat, updating the config per the issue, and dragging a folder over the new executable for three cases:

• SRT file with no styling (陰の実力者になりたくて！ Season 2, Episode 1)

  1
  00:00:09,225 --> 00:00:15,231
  （走る息遣い）
  
  2
  00:00:19,902 --> 00:00:22,905
  ハァハァ…。
  
  3
  00:00:22,905 --> 00:00:27,209
  （デルタ）アオーン！
  
  4
  00:00:40,256 --> 00:00:43,092
  （ベータ）先日のブシン祭の一件以来➡
  
  ...

  [00:00.50]ハァハァ…。
  [00:03.50]
  [00:03.50]（デルタ）アオーン！
  [00:07.80]
  [00:08.80]（ベータ）先日のブシン祭の一件以来➡
  
  ...
  

• SRT file with styling (陰の実力者になりたくて！ Season 1, Episode 1)

  1
  00:00:07,435 --> 00:00:12,482
  <font color="japanese">（目覚まし時計のアラーム）</font>
  
  2
  00:00:12,565 --> 00:00:15,110
  <font color="japanese">（アカネ）んっ んん…</font>
  
  3
  00:00:19,197 --> 00:00:20,323
  <font color="japanese">（アラームを止める音）</font>
  
  4
  00:00:20,407 --> 00:00:22,158
  <font color="japanese">（アカネ）ああ…</font>
  
  5
  00:00:23,326 --> 00:00:25,662
  <font color="japanese">そのまま寝ちゃったのか</font>
  
  ...

  [00:00.50]（アカネ）んっ んん…
  [00:03.04]
  [00:04.04]（アカネ）ああ…
  [00:05.79]
  [00:06.79]そのまま寝ちゃったのか
  
  ...
  

• ASS file with styling (からかい上手の高木さん Season 2, Episode 1):

  [Script Info]
  Title: [Erai-raws] Teasing Master Takagi-San S02E01 sdh-jpn
  ScriptType: v4.00+
  WrapStyle: 0
  PlayResX: 1280
  PlayResY: 720
  Video Zoom Percent: 1
  Scroll Position: 0
  Active Line: 0
  ScaledBorderAndShadow: yes
  
  [V4+ Styles]
  Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, OutlineColour, BackColour, Bold, Italic, Underline, StrikeOut, ScaleX, ScaleY, Spacing, Angle, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, Encoding
  Style: Default,Open Sans Semibold,45,&H00FFFFFF,&H000000FF,&H00020713,&H00000000,-1,0,0,0,100,100,0,0,1,1.7,0,2,10,10,15,1
  
  [Events]
  Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
  
  Dialogue: 0,0:00:01.63,0:00:03.22,Default,,0,0,0,,（学校のチャイム）\N（西片(にしかた)）うん？　うん？
  Dialogue: 0,0:00:03.34,0:00:05.68,Default,,0,0,0,,（西片）うん？　う〜ん？
  Dialogue: 0,0:00:06.64,0:00:08.85,Default,,0,0,0,,いやいや　まさか…
  Dialogue: 0,0:00:09.52,0:00:11.14,Default,,0,0,0,,（高木）ねえ 西片\N（西片）ウッ…
  Dialogue: 0,0:00:12.81,0:00:14.10,Default,,0,0,0,,（高木）何してんの？
  Dialogue: 0,0:00:14.27,0:00:18.57,Default,,0,0,0,,な… なに？　高木さん　別に何も
  
  ...
  

  [00:00.50]（学校のチャイム） （西片(にしかた)）うん？　うん？
  [00:02.09]
  [00:02.21]（西片）うん？　う〜ん？
  [00:04.55]
  [00:05.51]いやいや　まさか…
  [00:07.72]
  [00:08.39]（高木）ねえ 西片 （西片）ウッ…
  [00:10.01]
  [00:11.01]（高木）何してんの？
  [00:12.30]
  [00:12.47]な… なに？　高木さん　別に何も
  
  ...
  

To confirm the LRC output was as expected, and that SRT -> SRT didn't cause any issues. These were all video + external subtitle file cases.

I also noticed that lines containing only symbols like these from 葬送のフリーレン Season 1, Episode 1, seem to get removed now:

1
00:00:00,522 --> 00:00:05,110
{\an8}♪～

2
00:01:25,648 --> 00:01:29,903
{\an8}～♪

3
00:01:50,006 --> 00:01:52,926
（馬車の進行音）

4
00:01:59,891 --> 00:02:01,059
（ヒンメル）フリーレン

5
00:02:08,233 --> 00:02:10,443
{\an8}（フリーレン）
王都が見えてきたね

[00:00.50]（ヒンメル）フリーレン
[00:01.66]
[00:02.66]（フリーレン） 王都が見えてきたね
[00:04.87]

...

Whereas before the SRT file was just directly used so it produced:

[00:00.50]{\an8}♪～
[00:05.08]
[00:06.08]{\an8}～♪
[00:10.34]
[00:11.34]（ヒンメル）フリーレン
[00:12.51]
[00:13.51]{\an8}（フリーレン） 王都が見えてきたね

...

Even though I didn't modify the filtered_characters entry in config.json from this, which should have already been already been filtering out those lines (but wasn't, potentially because of the {\an8} formatting on the same line?):

"filtered_characters": "\u2669\u266a\u266b\u266c\uff5e\u301c",

Though ones only containing the {\an8} types of content that also have regular text are preserved (with the regular text only, not the {\an8}).