Variable substitution + refactor

[etene]

Hello,

I've implemented a feature I needed, variable substitution in commands. It essentially allows using variables like $ref or $sha in commands in the config file. They are substituted with values taken from the hook request payload.

I've also refactored the code a bit (a lot), and added tests run with tox. I'll probably make the Github CI run it later on.

It would be great if you could consider merging. Of course if it's not the direction you want to take your project to, no problem, I'll just keep my fork :) Also if you have suggestions, they're welcome.

Thanks !