Fixed a bug with images matching the wrong end punctuation

- `![alt-text]` would match 't' instead of ']' as the end puncuation of
  an image. Screwed up any syntax highlighter that highlighted the
  punctation differently than the name.
- Added support for `source.objc` and `source.objc++` because I used
  those and because https://github.com/jonschlinkert/sublime-markdown-extended/issues/16
  mentions it. It doesn't look amazing, but it's minimal enough that it
  doesn't break anything.