Add Gush::Job#skip!

[ace-subido]

As per discussion in #65 this pull request adds a Gush::Job#skip! that would mark a job as skipped, and returns from the point where self.skip! is called. After calling self.skip!, the workflow continues on with the other jobs.

Note: @pokonski was thinking of adding .skip_remaining! on another PR to keep the PR's small. What do you think? Or should I include that suggestion in this PR too?

[pokonski]

I tested it but have one note: could you add an integration spec for this case just to make sure descendants run?

Here: https://github.com/chaps-io/gush/blob/master/spec/features/integration_spec.rb

[natemontgomery]

@ace-subido I wanted to check in and see about updating this, it's something I think is worth implementing for sure! If you would rather not handle updating the branch I can hop on it, would definitely rather not duplicate a PR.

[ace-subido]

for sure! go ahead, I've not worked/looked on this for a long time and you may have better context.

[pokonski]

We will go with the refresh done by @natemontgomery , thank you for starting the work on this @ace-subido !