Added '- <UNTRACKED FILE>' tag to Branch info if current file is untracked
Borrowing tracking check function from https://github.com/epeli/vimconfig/blob/23d97ae6d668b22f3da56a5373744c9f579d117d/vimrc#L149-L164
Merge branch 'master' of https://github.com/johnathanz/vim-airline