Remove the --extension argument and set this value in pyproject.toml

[leouieda]

Description of the desired feature:

From #10, it became clear that we should passing this sort of option on the command line. Having it all in the pyproject.toml file makes it easier to run the tool since no matter what the project wants, it's always just burocrata FILE or burocrata --check FILE.

We should remove the option and create a new section in the pyproject.toml to add extensions. It would be a list of extensions and could be with or without the .. It should default to the same as the current default.

Are you willing to help implement and maintain this feature?

Yes but would love some help if anyone is up for it!