Add Shift+Tab map for tab complete in insert mode

Feature parity with `<c-p>` `<c-n>` tab completion duo.

vimrc

@@ -100,6 +100,7 @@ function! InsertTabWrapper()
     endif
 endfunction
 inoremap <Tab> <c-r>=InsertTabWrapper()<cr>
+inoremap <S-Tab> <c-n>
 
 " Exclude Javascript files in :Rtags via rails.vim due to warnings when parsing
 let g:Tlist_Ctags_Cmd="ctags --exclude='*.js'"