don't hard code list of checks, allow other modules/plugins to provide checks or reports

[jrglasgow]

While it is nice to have a Drush extension that doesn't require a module to be installed, or even to have the extension as part of the code base it would be nice to also allow modules in the code base, or other Drush extensions to provide checks and reports.

Would it be possible to add a plugin manager that could auto discover additional checks and reports?