Please add an `allow_fail` option

[gdubicki]

Is there an existing issue that is already proposing this?

• [X] I have searched the existing issues

Describe the problem imposed by not having this feature

Sometimes we merge a PR without waiting for its CircleCI workflow to complete. When we merge, we delete the source branch. Then the subsequent job in that workflow fails on the git-shallow-clone/checkout step because the branch doesn't exist...

We would like to have a solution to prevent such workflows from spoiling our CI statistics.

Describe the solution you'd like

I propose to add an allow_fail option, set to false by default. If set to true, then the whole step would exit with code 0 even if anything would go wrong in it.

We would set it for our non-critical jobs, such as sending test coverage info to our coverage server.