Conditionally override airline separators only if powerline fonts are not used.

The Airline separators were unconditionally overridden. I have conditionally overridden the separators only if the user doesn't use Powerline fonts.

User is expected to specify `g:airline_powerline_fonts=1` in `~/.vimrc.before.local` instead of `~/.vimrc.local`.
Merge pull request #441 from SaswatPadhi/Fix-for-overridden-airline-separators

Conditionally override airline separators only if powerline fonts are not used