feat(specs): Introduce 'improvement' type

[damianopetrungaro]

@stevemao let me know what do you think.
I saw in another PR that we'll use noun instead of verb, so improvement is used instead of improve

[damianopetrungaro]

@stevemao @bcoe take a look here

[ungoldman]

I think this concept is already covered by refactor (see https://github.com/angular/angular.js/blob/master/DEVELOPERS.md#type), which should not trigger a version bump.

[damianopetrungaro]

Refactor means no breaking changes are made.
Improvement may break APIs but it's not a feature neither a fix.

[bcoe]

@damianopetrungaro sorry for the slow turnaround, don't have a ton of time for open-source these days unfortunately.

There's nothing in the spec that stops you from using the improvement verb; but functionality it's fairly similar to feat and I think at this time I'd like to avoid adding it to the spec itself.

If you'd like to add it as a recommended verb to the Others:, perhaps we can make the recommendation here.