Fix conditional in vimrcs/extended.vim

In the font section change has("linux") to has("unix").
Vim does not have "linux" feature only "unix".
Updated all the plugins. Removed powerline. Added vim-airline (replacement for powerline). Added vim-fugitive.
Merge pull request #28 from CrypticStranger/fix-os-conditional

Fix conditional in vimrcs/extended.vim
Update README.md

Updated link to ack.vim