Add support for stack tags

[mdub]

CloudFormation supports stack tags, which are applied to the stack itself, and resources within it:

• http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/cfn-console-add-tags.html

Stackup should support setting, updating and querying stack tags.

[mdub]

This is underway, with support for :tags in Stack#up (9853424227badad29b1eb66ba6ac893fd621871d) and a Stack#tags accessor (a4d535ba1f07e1d4138c6e6ac538193f2c38ba92).

I'm wondering how best to support tags via the CLI. One option is go with a file, like we do for template and parameters:

stack mystack up -t template.yml --tags tags.yml

Another is to support key=val options:

stack mystack up -t template.yml --tag owner=mike --tag environment=prod

I'm leaning towards the file option for now.

[mdub]

I'm calling this done, with f0d0d8c.