Fix some bugs in grammars.yml

There were two bad bugs in grammars.yml:

0. The scope name for Markdown was wrong, preventing Markdown from getting syntax-highlighted
0. The scope name for WebIDL was wrong, masking the fact that it conflicts with the scope name for IDL (/cc https://github.com/github/linguist/pull/1850#discussion-diff-22003882)

I've fixed the scope name for Markdown, and removed the WebIDL grammar for now. I also cleaned up the file a little bit, and added a couple of tests to help us keep it clean in the future. Unfortunately the tests wouldn't have caught the two issues described above. This makes me think that we should probably switch to using submodules for grammars where possible and remove those grammars from grammars.yml.

/cc @arfon @pchaigno