Allow locale to be a list

[benjaminwinger]

This allows an arbitrary-length of fallback chain for the user-requested locale, letting users provide multiple preferred languages instead of just one.

[benjaminwinger]

Updated to include tests.

Note that I removed the following test, as there doesn't seem to be a meaningful result for it with arbitrary length fallback chains, and I'm not sure that it's necessary in the first place (en should generally only contain messages which are shared between the country-specific locales, and users should usually request a country-specific locale).

it("returns the fallbackLocale ancestry if locale is parent of fallbackLocale", function()
     assert.same(variants.fallbacks('en','en-US'), {'en-US', 'en'})
end)