Support using other labels as criteria

[bkimbrough88]

This action so far has been based on using the file path(s) to determine which labels to set, but would it be possible to also have it support labels that are already on the PR? My use case is that I want to add a label if the VERSION file has not been modified. However if the PR has the label skip-release, that file is no longer required to be modified, thus no additional labels should be produced.

One potential way for this to work would be something like this:

needs-version-bump:
  - all:
    - "!VERSION"
    - "!label:skip-release"