feat: support custom config path

[jimschubert]

This adds a new input item config_path. It'll be released as v3, after I update the docs and workflows to better orchestrate the releases.

Fixes #7

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[anthonyalayo]

@jimschubert can this also be used to make the labeler.yml work within a branch instead of master only? Adding it to a repo for the first time in a PR, I get this error:

404 Not Found []"