Add support for indie-config as an alternative/additional source of configs

[voxpelli]

As pointed out in #89 the current localStorage might not always be fully reliable and when #89 is solved so that a UI is still showed it will be fully anonymous.

It would be interesting then to support the IndieWeb "polyfill" for configurations, indie-config: https://indiewebcamp.com/indie-config

It's built to supply endpoints for buttons similar to SubToMe's subscribe button through what the IndieWeb community calls webactions: https://indiewebcamp.com/webactions But it's an independent mechanism that can be utilized for any similar purpose.

By including an indie-config loader such as mine (https://github.com/voxpelli/voxpelli.github.com/blob/master/js/indieconfig.js) one could progressively enhance the current experience to present a users personal subscription service choice as one of the subscribe options – and for users without any such choice set up the experience will be exactly the same as today – they won't notice any difference.

I could try to find time to look into making a PR of this if it would of interest.

[julien51]

Ho yes! Please, submit a PR for this! Progressive enhancement FTW!