Better file completion

Better file completion for SudoWrite and SudoRead commands. closes
github issue #20 (reported by Daniel Hahler, thanks!)
Don't twice escape the directory

Mkdir already is already called with an shellescaped parameter. No need
to quote it a second time for the Rmdir() call on VimExit.

Closes issue #22, reported by Daniel Hahler, thanks!
handle s:error_file consistently