feature: autocompletion for the tasks

[tunnckoCore]

for zsh and bash

[jonschlinkert]

I have some code for this locally, based on gulp. k, marking as a request, thx

[JelaniThompson]

@jonschlinkert Hey are you still working on this? If not, I could definitely tackle it!

[jonschlinkert]

I could definitely tackle it!

that would be great! I think we should be able to use https://github.com/gulpjs/gulp-cli/tree/master/completion as a starting point (if we do make sure to give credit :)

or however you want to do it! thanks!