[WIP] add custom checks to brakeman itself

Started hacking on this a bit. I stole all the logic from @gregose's sentinel stuff. I changed up the metaprogramming a bit and tried to make the API for creating new checks such that it could be defined in YAML. I'm going to keep hacking on this here and there, but I wanted to throw this up to get some :eyes: from @gregose and @ptoomey3. The thought would be to try to get this upstreamed once its finished.

* [ ] Configurable severity
* [ ] Add CLI options for specifying a config file
* [ ] Add config file parsing
* [ ] Write tests :frowning: