Separate keymaps to specific platforms, and make OSX play nice

On OSX, `ctrl+f2` focuses the menu bar, and the built-in keybinding for toggling bookmarks in Sublime is `super+f2` anyway, so this makes the package work better as a built-in bookmarks replacement for OSX.

I don't know if the natural binding on Linux is different from Windows so I left that as-is. But, the file is there if someone had a strong opinion about it.