Add plain C language mode

I mostly used the existing CPP language definition as a base, although C has its own set of keywords and built-ins. It _is_ a different programming language, and as such would benefit from having a definition in highlight.js.

This would be particularly important in cases where the highlighting language is known beforehand (and is not being auto-detected). In my case, I had a code snippet with class="language-c", and not knowing otherwise highlight.js was unfortunately guessing that the snippet was Haskell.