Use singular form for metadata properties

[Mottie]

I made a few updates to the spec & database:

• Changed continent to only allow an array instead of both a string and array to make accessing the values consistent
• Updated the wording on how to name a variant file. It now explicitly states that the extlang subtag of an IETF language tag should be used to name the variant file.
• Renamed sources to source to be consistent.
• Renamed countries to country to be consistent.

It doesn't look like the API will need to be modified for these changes.

[Mottie]

Updated... I didn't see any place in the API that needed updating.

[julkue]

I didn't see any place in the API that needed updating.

I'll double check this asap.