cli: support for config files

It would be great if config files would be supported, e.g. to ignore/whitelist terms (#114, https://github.com/btford/write-good/commit/685c0a24a490c13433a279cf1a1500e0cbcff992).

It would be useful if it supported an explicit config file via an argument, and otherwise look for a config starting from the linted file upwards, falling back to a global config in the user's home (preferably XDG-style, i.e. `~/.config/writegood/config`).

I assume / hope there a good libraries for this already?

As for the format: it would be nice if this wasn't in JSON, so that it would allow for comments in it.