[WIP] Add custom bash completion

[eddiezane]

Figured out how to implement this. It required updating Cobra.

Adding the remaining resources won't be too difficult - just tedious.

Still need to figure out how to handle nested completion like:

doctl kubernetes cluster node-pool get <cluster-id|cluster-name> <pool-id|pool-name>

Closes #699

[andrewsomething]

We check the vendor/ directory into source. Can you run make vendor and commit those changes?

[chrisshroba]

I see that Cobra supports auto-generating completion functions for multiple shells (bash, zsh, fish, powershell). Would you consider modifying to use that method, so we get completion in multiple shells for free? If not, I can try my hand at it.

[eddiezane]

@chrisshroba please feel free to take over!

It will be good to hear your experience working with the new Cobra features. We've been looking into this for kubectl.

Hopefully some good guidance for you here:
kubernetes/kubectl#882
kubernetes/kubernetes#93714