Add support for Textile (markup format) files

[hoijui]

Comment style for these files is documented here:
https://textile-lang.com/doc/textile-comments

They have a somewhat non-standard way for how their native comments work:

• ###. starts a multi-line comment that ends with an empty line
• ###.. starts a multi-line comment that ends with a for a new paragraph
• ... but they also support normal HTML comments: <!-- BLA -->

Would it be fine if I'd add the .textile suffix simply with HtmlCommentStyle?
or should I rather implement a comment style that supports all three?
I can't say when I'd find the time for that.

[mxmehl]

If we can reuse an existing comment style, I'd personally prefer that. It only requires two new lines then:

1. Adding the file extension to the list
2. Adding a changelog entry ;)

[hoijui]

will do then!