Setup for a future with multiple possible data sources

[fershad]

At this time, the project is being built to use the Electricity Maps API](https://api-portal.electricitymaps.com/) to fetch current grid data for regions around the World. However, we understand that choice is important, and want it to be possible for other data sources to be added to this project as it develops.

Desired state

It should be possible for a user to specify the data source the want to use for fetching grid data. This would most likely be through an options object initially, however adding a config file later (#5) might help streamline that.

It should also be possible for data providers to write & contribute their own integrations for this library. At this time, it is planned that integrations for data sources remain within this library.