check for existance of autocmd group before undefining it
handle cases where the augroup has not be defined yet. resolves #492
add support for configurable whitespace checking algorithms and default it to the simplest one. resolves #484
provide option to disable runtimepath extension loading. resolves #483
Enable :AirlineWhitespaceToggle,

even if g:airline#extensions#whitespace#enabled is disabled
Merge branch 'master' of github.com:chrisbra/vim-airline