Move community stats command to a new plugin

Somewhat related to https://github.com/coala/corobo/issues/357 , the `stats` command should be a separate plugin, and it should work for people using a token which does not have write access to the repositories.

The rest of that plugin is about enabling processes, relaying operations to the server on behalf of users.

The `stats` command is not related to that, and should be in a different plugin. Non-coala users of corobo may not like our processes (assign/invite/etc) and wish to disable that plugin. But they may be interested in a org analytics overview plugin.

Read related issue #357, then locate the `stats` command and the current plugin boundary. Separate the command into its own plugin, keeping it usable with tokens that lack repository write access. Done means users can disable the process-related plugin while retaining the organization analytics overview.

Written by the indexing model from the issue text.