Ability to use web-ext config files

[JessFairbairn]

Hi, just discovered this action and very thankful for it!

Web-ext supports configuration files for things like "ignored-files", I'd like to be able to point this integration to that rather than have to store the ignore files list in two places?

[kewisch]

This sounds like a great idea! It might not be 100% straightforward to implement, as the config file format relies on a yargs argv object to load the commands, and my action is calling the commands directly.

We might be able to fake a program as in the tests: https://github.com/mozilla/web-ext/blob/354aa67c97dcfd02f9ae3046932206680d8dea23/tests/unit/test.config.js#L50

And here is another good starting point: https://github.com/mozilla/web-ext/blob/354aa67c97dcfd02f9ae3046932206680d8dea23/src/config.js#L38

If you are configuring everything with the config file anyway, you might get away with just using run to call web-ext directly.