Remove or extract doctor command

[jaredcwhite]

Once of the goals of Bridgetown 2.0 is to streamline the codebase and improve maintainability, outside of key functionality site authors rely on. I must confess I've personally never used the doctor command on any of my sites, and I don't fully understand its rationale. (Seems very much a holdover from past Jekyll days.) Unless there's push back to the contrary, I'm thinking of either extracting it out to a separate community-maintained gem or removing it entirely. I do like the idea of including more diagnosing/troubleshooting functionality in Bridgetown, but I think it makes more sense to be part of the site build process via flags/settings in development, rather than a specific command.