Should have an option to ignore front matter

[sylvestre]

When we have some md files also used for Jekyll, they come with a Front Matter.
It would be great to be able to tell recommonmark to ignore the declaration as it is currently rendering.

Here is an example of a rendering without this option:

[sylvestre]

See also: readthedocs/commonmark.py#208

[sylvestre]

I have been able to workaround this issue by using https://github.com/eyeseast/python-frontmatter/
to extract the relevant info from the fm and rewrite the md file. See:
https://hg.mozilla.org/mozilla-central/file/tip/tools/docs/moztreedocs/__init__.py#l127