insert mode <Tab> indents or completes

Taken from Gary Bernhardt
https://github.com/garybernhardt/dotfiles/blob/833c882b/.vimrc#L141-154

vimrc

@@ -159,6 +159,18 @@ let g:ackprg = 'ag --nogroup --nocolor --column'
 " In command-line mode, C-a jumps to beginning (to match C-e)
 cnoremap <C-a> <Home>
 
+" <Tab> indents if at the beginning of a line; otherwise does completion
+function! InsertTabWrapper()
+  let col = col('.') - 1
+  if !col || getline('.')[col - 1] !~ '\k'
+    return "\<tab>"
+  else
+    return "\<c-p>"
+  endif
+endfunction
+inoremap <tab> <c-r>=InsertTabWrapper()<cr>
+inoremap <s-tab> <c-n>
+
 " ignore Rubinius, Sass cache files
 set wildignore+=tmp/**,*.rbc,.rbx,*.scssc,*.sassc
 " ignore Bundler standalone/vendor installs & gems