Bug fix and improvement

Hi,

 * The Fix 'Diff with previous window' did not work. I made a new one. I tested on `st2` and `st3` on Window. Hope it will work on OSX.

* I fixed the Issue #44 by adding `expand_full_file_name_in_tab` parameter to the config to be more flexible.

* I hide items in context and tab menu when features cannot be used for more clarity.

Thanks