Acting on tagged droplets

[tfogo]

Can we add the ability to run droplet actions on droplets by tags? So the usage would be:

$ doctl compute droplet-action <droplet-action> (<droplet-id> | --tag <tag>)

In the case both a tag flag and a droplet ID arg are provided, the CLI would throw an error asking for only one option.

If this usage looks okay I'm happy to submit a PR to implement it.

[xmudrii]

I've added needed calls for this in #202 to doctl/do, but never found the time to actually implement commands for this. I believe this could be very helpful, but it's important to find the right usage pattern.

In my opinion, that one you mentioned (<droplet-id> | --tag <droplet-tag>) should be good enough, but I would love to hear about this from maintainers.

[tfogo]

Thanks @xmudrii! @mauricio do you have any thoughts on the usage here?