Plugins should be upgradable (semi)automatically

[hyfen]

If a user upgrades chronicle-etl and then uses a plugin, the plugin's gemspec might resolve an older version of chronicle-etl and then a ConflictError will be raised when attempting to load it.

This should be pretty recoverable: we check if a plugin is installed but incompatible and then call Gem.install again (todo: figure out if we have to pass an explicit version).

Also, would be useful to have a $chronicle-etl plugins:update_all command. Also maybe a $chronicle-etl plugins:cleanup for removing old versions.