Add FreeFEM lexer (inherited from C++)

[sgarnotel]

Lexer for FreeFEM, following the recommandations of PR #1020

[pyrmont]

@sgarnotel Sorry, this took a lot longer to get to than I originally intended! I've made some minor (mostly stylistic changes). Let me know what you think!

[sgarnotel]

@pyrmont It is ok for me.
On the other hand, in the demo, I wanted to test the ' operator, but it is not such important.
Thanks

[pyrmont]

@sgarnotel The demo is really just to make sure that the lexer loads. It's not really intended to be used for testing of the lexing of particular constructs.

If you want to do that, you can either add the construct to your visual spec and then check it visually or you can add it to the test file for the particular lexer. You can see an example of how that would work in the recent PR for the Console lexer.

[sgarnotel]

@pyrmont Ok, so all is ok for me

[pyrmont]

Cool. I'll merge this now and it'll be in the next release of Rouge coming 14 January.