Change file type canonicalization to treat C as C++

Please refer to the inline comments for more details.
This will hopefully fix issue #91 on GitHub:

  https://github.com/xolox/vim-easytags/issues/91

PS. I've completely removed the objc/objcpp grouping because I don't know
    these languages so I don't know what grouping would be correct anyway.
    Until a user with an informed opinion chimes in it's better to just
    remove this grouping.