Add mappings for syntax toggling

[dmitrivereshchagin]

One may use this mappings due to performance reasons. In addition sometimes there is a code that is much easier to read with syntax highlighting turned off.

[auwsmit]

here's a little vimrc snippet for anyone who comes across this and wants a simple fix:

" toggle syntax highlighting
" useful when Vim is crippled by its inefficient highlighting
fun! s:ToggleSyntaxHighlighting() abort
  let folds = &foldlevel
  execute 'syntax '.(exists("g:syntax_on") ? 'off' : 'enable')
  let &foldlevel=folds
endfun
command! ToggleSyntax call <SID>ToggleSyntaxHighlighting()
nnoremap yoS :call <SID>ToggleSyntaxHighlighting()<CR>