Feature: Don't force users to update the plugin list on update

Previously, when a new plugin was added, the users of Shellwreck would need to
know about it and add it to their plugin list.

With this new feature, everyone's configuration starts with a 'base' plugin
list, which is stored in version control and can be updated by the developers,
but, if the user wishes, they can remove or add additional plugins from that
list.

As a nuclear option, we are still allowing the user to specify an absolute list
of plugins.

All of this has been documented in the zsh and bash personal rc templates.

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Github Issue URLs:
  * https://github.com/jfelchner/shellwreck/issues/7
Actions:
  * Closes #7
Branch:        feature/do-not-make-the-user-update-the-plugin-list-when-new-ones-are-added
Signed-off-by: Jeff Felchner <accounts+git@thekompanee.com>
Merge branch 'feature/do-not-make-the-user-update-the-plugin-list-when-new-ones-are-added-closes-7'