Support reading config from ignore file

Is your feature request related to a problem? Please describe. We run Brakeman with all checks enabled, which is not the default so either people have to always remember to pass --run-all-checks or we need to use a config file.

This means we have both a config/brakeman.yml and a config/brakeman.ignore file, when we're just configuring one option.

Describe the solution you'd like It would be nice if we could specify config options in the ignore file, rather than needing a whole second file.

I'm not that fussed about the specifics in regards to e.g. if the "ignore" process writes the config properties automatically, or if another term is used since its no longer just "ignores" (though I would like them to live in a default i.e. if the ignores are moved into the config/brakeman.yml, then I'd not want Brakeman to continue defaulting to config/brakeman.ignore), and so on.

Describe alternatives you've considered

Having the two files

Additional context

My thinking behind this is that brakeman already loads the default ignore file if it exists, and that file is JSON with an object as its root so it shouldn't be breaking to add new properties at the top level.

I also feel like a lot of the potential questions and concerns could be addressed by saying "yml configs take priority and should be used for advanced stuff e.g. adding comments; this is just a convenience"