Plugins should be able to report their connectors without needing to be required

[hyfen]

As identified in #24, activating all installed plugins at the same time will lead to dependency problems.

We often need to know which connectors are available (eg. $ chronicle-etl connectors:list) so we need a way for plugins to report their available connectors without the gem being activated/required.

A few options:

• use the gem's metadata fields
• requiring a special plugin.rb file that doesn't load any other dependencies
• ???