Fixed a bug with images matching the wrong end punctuation

This is kinda two things in one.
- 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 (because ST only minimally supports objc), but it's 
  enough that it works visually and doesn't break anything.