Multi-line comments

[mernst]

Currently only single-line comments are supported.

This is a feature request to support comments that span multiple lines, such as with

<!--
...
-->

Note that <!-- ... --> within fenced code blocks is not a comment.
A fenced code block is text within triple backticks:

```sh
here is fenced code
more fenced code
<!-- The pull request shouldn't have any effect

on this comment. -->
```
<!-- The pull request should have an effect

on this comment. -->

This requires changing the command-line arguments. For example:

OLD:

--comment-re=string. Matches an entire comment. [default: ^%.*]

NEW:

--comment-line-re=string. Matches an entire comment line. [default: ^%.*]
--comment-multiline-start-re=string. Matches the start of a multiline comment. [default: ^<!--]
--comment-multiline-end-re=string. Matches the end of a multiline comment. [default: ^-->]

This change actually needs to be done in EntryReader in plume-util, but the lookup program needs new command-line arguments to pass information to EntryReader.