Allow template override, misc improvements

[timkelty]

Added some features and improvements:

• Added a template override setting, so users can use their own template/fields to render the contents of the pages if they wish.
• Added a plugin name override setting, so users can change the name of the plugin in the nav (e.g. Docs)
• Store section settings ad IDs instead of handles, to prevent breaking when a section handle is renamed
• Renamed 'channel' settings to 'section'
• Show a default message if the body field doesn't exist
• Only show sections with hasUrls in the settings, as it is required they have urls for the plugin to work.
• Allow config file overrides for all settings, display warnings and disable inputs on settings page when overrides exist

...and I apologize for the big PR! Great work on the plugin, though.

[RobErskine]

Dang dude! This is amazing. Reviewing now, and will merge in soon.

[timkelty]

No problem, was gonna just hop in for a quick fix and got carried away :)