Add OS-specific keybindings to avoid clobbering ctrl+J on Win/Linux

This pull request moves `Default.sublime-keymap` to `Default (OSX).sublime-keymap`, and adds `Default (Windows).sublime-keymap` and `Default (Linux).sublime-keymap` mapping alt+J instead of ctrl+J, since on those platforms ctrl+J is already bound to join lines by default.

It'd also be feasible to just change the default across all platforms to alt+J instead, but that'd mean changing the default on OS X that people are already used to, and technically option+keys tends to insert other characters on OSX I suppose.  Up to you whether you'd rather do that instead though.