First line indent

[jdutant]

Not sure whether this filter is simple enough for this repo.

The filter provides finer control of the first-line indentation style of separating paragraphs in Pandoc's markdown, targetting HTML and LaTeX/PDF outputs. It switches on Pandoc's first-line
indent style, automatically removes (typically unwanted) first-line indents after blockquotes and
lists unless the user specifies otherwise, and allows users to control whether a paragraph has first-line indentation in both LaTeX and HTML by inserting \indent or \noindent at the beginning of a paragraph.

Limitation: at present it only handles English-style typography in HTML (in French and perhaps other languages, paragraphs are first-line indented even after headings; this is handled in LaTeX by the babel or polyglossia languages, not re-implemented for HTML in that filter.)

[tarleb]

Looking great, thanks Julien!