add support for JavaScript

This commit looks bigger than it actually is because I have Sublime set to remove trailing whitespace. Sorry 'bout that. It's really a single key added to the sublime-keymap file. It's identical to the other ones ("super+option+a"), except that the operand is "source.js". I added a single three-line function to clang_format.py called is_supported(), that compares the current syntax with 'C.tmLanguage', 'C++.tmLanguage', and 'JavaScript.tmLanguage'. A three-line function and a single key in the keymap is all it took to support JavaScript. I think adding JS support will increase the popularity of your plugin, when more people discover that clang-format works on JS files.