Make ...#excludes|exclude_preview runtime configurable
Make ...#excludes|exclude_preview runtime configurable

airline#extensions#tabline#excludes and
airline#extensions#tabline#exclude_preview previously had no impact if
changed after vim load.  This fixes that.

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