Simplified `supportLngs` back to its original values

`options.supportedLngs` now takes only an array of locales as an
argument (as it originaly did). Any other value will have the same
effect as leaving it undefined.

Put back the `options.supportedLngs` default value assignment line in
`i18n.init()`, which I had accidentally removed from my last commit.

Renamed my helper functions with more accurate names.

Replaced `_extractLocales` with express' `request.acceptedLanguages` as
suggested by @Djebbz .