Update signs both on FocusGained and FocusLost

This pull request changes `g:signify_update_on_focusgained` to `g:signify_update_on_focus` which refreshes signs both on `FocusGained` and `FocusLost`. I think it makes sense to do it since unfocused MacVim still shows signs which might be confusing.

On the other hand, it might be undesired behavior. In this case I think it might make sense to expose public API so users could add their own autocommands. I can do this if that's preferrable.