Add integrations with national/regional/local grid data APIs

[fershad]

Is your feature request related to a problem? Please describe.
This isn't related to a problem, however it would be a follow on from #134. It would allow for an ecosystem of data plugins to be built for use in CO2.js that would allow developers to access data for specific energy grids (national, regional, local).

Describe the solution you'd like
Developers would be able to import an appropriate plugin that can be used for fetch grid intensity from a regional API. That data would be returned to CO2.js and exposed via a set of pre-defined (yet to be defined) APIs that would provide a standard way for developers to interact across plugins.

Describe alternatives you've considered
This feature is primarily focused on developers who want data for a single location. Developers could write their own code to fetch data from any API, though this would be non-standardised and time consuming. Alternately, they could use services like Electricity Maps or WattTime to get this information for a specific location however there might be no guarantee that data is available through those services or it may end up being prohibitive cost-wise for the developer.

Additional context
Initial candidates for implementation are:

• The UK National grid (https://api.carbonintensity.org.uk/)