define highlighting group, if it not exists
define highlighting group, if it not exists

previously, it could have been skipped, if the old highlighting
attribute was the same as the current one. However, if the group does
not exist, it should still be defined

closes #1404

HarmtH/vim-airline,yzhou61/vim-airline,unphased/vim-airline,tschundler/dotfiles,tschundler/dotfiles,tschundler/dotfiles,rchicoli/vim-airline,bling/vim-airline,chrisbra/vim-airline