Add certificate install and uninstall commands

[swalkinshaw]

trellis certificate install will install the root certificate from a Trellis development VM (by default) into your computer's system truststore. This means local development HTTPS sites will be considered secure by web browsers and won't show insecure warnings.

trellis certificate uninstall removes the previously installed certificate from your computer's system "truststore".

By default this integrates with Trellis and relies on the changes in https://github.com/roots/trellis/pull/1310. However, by passing the --path option this is a useful standalone tool to trust any root certificate on your computer.