Feature request: Ability for services and plugins to add settings.

[Onfire7]

It would be great if services and plugins could add their own settings (API keys, default service settings, etc.), instead of having to rely on a config file.

What, ideally, should this look like?

Right now I am thinking a "Settings" button that shows up next to "Add New Service" only if there are any plugins that define settings.

On that page would either be tabs or sections for each service/plugin that defines any settings.

This could probably be very similar in implementation to #68 to allow for plugins to define new fields for the service page.

I'm thinking of taking this (and #68) on in the next few weeks if it would be a welcome feature. Thoughts?