Style fixes (via vint) (#662)

This also adds a augroup for `jedi#add_goto_window`.

plugin/jedi.vim

@@ -4,10 +4,12 @@
 " This part of the software is just the vim interface. The really big deal is
 " the Jedi Python library.
 
-if !exists("g:jedi#auto_vim_configuration") || g:jedi#auto_vim_configuration
+if get(g:, 'jedi#auto_vim_configuration', 0)
     " jedi-vim doesn't work in compatible mode (vim script syntax problems)
     if &compatible
+        " vint: -ProhibitSetNoCompatible
         set nocompatible
+        " vint: +ProhibitSetNoCompatible
     endif
 
     " jedi-vim really needs, otherwise jedi-vim cannot start.