read config from file

[dudicoco]

Description: It would be very helpful if the action could read its config from a file instead of passing variables.

Justification: Probot stale uses a stale.yml file to read its config, it's easier for users migrating from the bot to the action to keep using the same file. Also in general the config in the file is easier to work with than action variables.

[dsame]

Hello @dudicoco , thanks for your input. We are starting to investigate the issue