Investigate how to make Reviewcheck scriptable

[plaos]

There's already an issue for making the output/UI scriptable, this is more about figuring out what needs to be done to make Reviewcheck ergonomic to use in a shell script. Things like return codes, silent operations, filtering by "check type" (author, pinged, etc.) should be considered. A comment on this issue explaining what should be done is enough here, as well as creating new issues.

One (maybe goofy) idea is that the return code of the program can the same as the amount of open threads. Perhaps not as default, but as a flag?