WIP: Exclude init initializer from plugins list command count

[akarzim]

This is a 🐛 bug fix.

Summary

This PR excludes init (Initializer) from the count of registered plugins when the bin/bridgetown plugins list command is run. See #770 for details.

Context

Fixes #770

[akarzim]

@jaredcwhite I put the tests in a separate commit and this PR in WIP because I found myself unable to set up a valid site configuration to test the CLI commands :/ What could I have missed? Could you point me in the right direction?

[jaredcwhite]

I'll try to get this over the hump for v2.0. Apologies for the long delay!