Add support for branch argument

Added option for branch argument ex. `yo evolved @branch-name`. Supports branches, git tags, and commit sha.

### Changes

* Added argument checker for `@` key
* Added branch verification from Github archive
* Added 'latest' check when no branch is passed

### Testing

* Run `yo evolved @v1.0.0` and child theme should match v1.0.0
* Run `yo evolved @v1.1.1` and child theme should match v1.1.1
* Run `yo evolved` and child theme should match latest release (currently v1.1.1)

### Review

@germanny 
@ericrasch 
@jameswlane 
@ericclemmons

### Notes

* When calling `master` yeoman was grabbing the locally cached version even if there had been updates
* Adding branch argument support allows easier testing of theme PRs
`this` here refers to the Generator so the `args` object is any arguments passed when initially calling `yo`