Feature Request: Ignore Branch

So the idea behind this is that the pipeline will run for any branch except for the one(s) specified. Then, theoretically, this can be set globally so that all pipelines are non-production by excluding the "master" branch.

```
Example:
resources:
- name: source-code
  type: git
  source:
    uri: git@github.com:example/example.git
    except_branch: master
```