Add support for central translation repository

[filips123]

It adds support for central translation repository and fixes #1261.

Translations are saved into src/locales/{lang}.js (where {lang} is IETF BCP 47 language tag) and bundled to Plyr file.
Language could be specified with language options and it could not be changes by user (for now).
Translation could still be overwritten by i18n option.

In the future, you should add support for user changing of language in UI options. I've already added language name field for this (in src/locales/index.js). See #1270.

You should also find translators to add translations. I added translation guide to contributing.md and I could translate UI to Slovenian.

[sampotts]

Perhaps we just create a build task that accepts an argument for an ISO code and bundles that lang (if it exists) with the rest of the JS.

[filips123]

Perhaps we just create a build task that accepts an argument for an ISO code and bundles that lang (if it exists) with the rest of the JS.

Unfortunately I can't create custom build tasks because I (currently) don't know much about Node.js build systems and Gulp.

[filips123]

From #1269 (review):

Also should we not use a config option for the default? It may not always be 'en'.

[filips123]

Default language should be en because it is only for times, when translation doesn't have some translated strings, so like fallback language.

But maybe there could be support to change fallback language.

[filips123]

@sampotts Ping.

[garbinmarcelo]

any news? thank's