Improves tag handling for droplet create

[akshaychhajed]

Fixes #127
Runs check to ensure tags exist before creating droplet

[xmudrii]

Looks good.

But what about creating tag alongside Droplet? That can be done automatically, when you try to create Droplet with new tag, create tag too. Or ask user to create tag/ignore/cancel creation.
I personally I would go automatically, as I think less interaction == better, especially because doctl is made for scripts integration and so on.
Probably @bryanl can give us his opinion on this topic. 👍

[mchitten]

This looks fine. Today we implemented tags in droplet create, which does essentially what @xmudrii is suggesting (creating a tag and applying it to the droplet during create). That is implemented in #165.

For this, 👍 .

[nanzhong]

lgtm 👍