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 :AirlineToggleWhitespace,

even if g:airline#extensions#whitespace#enabled is disabled
Merge pull request #493 from chrisbra/master

Enable :AirlineToggleWhitespace,
append to the existing value in case the user defined it.