Adding a label should remove all other labels of the same color

[jtuttle]

In order to not confuse Waffle about a story's state, any labels added by dev-flow will have to replace existing labels of the same color, following the Highlander principle. This can be an opinionated part of the workflow that is enforced through documentation.

[jtuttle]

We should almost definitely do this when applying labels as part of a command (like in start, codereview, and complete) since an issue shouldn't exist in multiple states simultaneously. It should probably be controlled with a --replace flag for the df label command so that it remains generically useful for applying labels.