`miqbot` CLI

It would be nice for developers to be able to run the same linters that the bot does from their own machine via a miqbot CLI command, or some other form.

Some considerations:

• Making it a gem would allow for an easy install, but I think based on some issues with including it in a bundle, it is preferred to have it as a separate install. We could probably just add a miqbot.gemspec that pulls only what we need from the lib/ dir to make this happen.
• Might want to make some changes to the worker classes so that the same code paths they call into are what is being called in via the CLI. I don't for see this being a lot of work, just want to make sure that we are supplying methods for the classes in lib with non-activerecord objects and only the service objects as input. This might already be the case, but I would have to check.
• The "public configs" notion on https://github.com/ManageIQ/miq_bot/issues/486 would be relevant for this if there is anything that is "config specific" that we would want developers to have when running locally. Not sure there is, but would have to check.
• pronto is already CLI based, and there was an effort a while back to make use of this instead of our custom code (see #406, #412, and #424 for details). This probably wouldn't replace the need for a custom miqbot command, but it would greatly simplify the glue needed for that, and we probably could just inherit a lot of things from that project for our implementation.