Added 'goto' shortcut in the sublime-keymap.

The 'goto' shortcut is really handy, and it avoid people to wander through github to find it !
I didn't try the shortcut on Mac OS or Windows, but they should work.

Personally I don't like the F2 shortcut cause this touch is toooo far away on the keyboard for file navigation. But that's a matter of taste.

Also as mentioned in another PR, the correct command is "goto_previous" and not "goto_prev".

Anyway, thanks for this great plugin which should definitively be included in the official sublime version :-)