Updating plugin(s) could install missing plugins

[craigfurman]

I think it would be quite useful if when updating plugin(s), missing plugins were installed. This would simplify code that configures workstations, as only one command would have to be issued (bin/update_plugins all), instead of two (bin/install_plugins, and bin/update_plugins all).

Would you be interested in a PR that implements this? I'm happy to make one if so.