Newlines fix, add support multiline labels

[GrigoriiTarasov]

Currently first newline is ignored and 2nd and more breaks background both for content and label (with or without spaces between)

[Pic1 Example from #21 few text old/current version]

This commit adds the argument "newline_mode"
with options:
1) 'old' (default) - As is now: ignores first consequential \n, breaks background on second. Nothing changed.

[Pic2 Lots of text old/current version]

2) 'flex' - Writes all newlines. Background fixed for both label and content. Each annotation is grouped in box having same label on the side (as side effect of display inline-flex preserved)

[Pic3 Few text flex option]

[Pic4 Lots of text flex option]

3) 'multiline' - writes all newlines with proper background for content. Lines are preserved (by display inline instead of inline-flex).

[Pic5 Few text multiline option]

[Pic6 Lots of text multiline option]

Default behavior is the old one thus it backward-compatible.
It resolves #21 and gives newline handling in unified and predictable fashion.

[fkdosilovic]

This would be a great fix for the component. (Just leaving a comment to show the importance of the fix for the community :) ).

[blackary]

LGTM -- what do you think @tvst?

[feljost]

@tvst @blackary I would greatly appreciate this feature in a new release. If possible, could this be merged and a new pip release be made?