Increased constrast for selected OmniComplete item

In a 256 color terminal the selected entry when using OmniComplete is
barely readable against the light grey background. Tested this under
both xterm and xfce4-terminal. To fix this the foreground for PmenuSel
is set to ctermfg=253.
Merge pull request #18 from octol/master

Foreground/background contrast of the OmniComplete menu for 256 color terminals.