Added lang_only as new langTagResolver

[antony-k1208]

Added possibility to be able to get only the lang tag as result, in order to limit the language files to one for each language, i.e. locale-de.json, locale-en.json. Especially useful when using the staticFilesLoader.

[knalli]

@ThinklabsGbR

Thank you for contribution. Please follow https://github.com/angular-translate/angular-translate/blob/master/CONTRIBUTING.md which contains some notes about contribution steps in this project.

Particular in this case, I remark the followings:

• apply a proper commit message (see in guideline about conventional changelog)
• add a test (for the other resolvers they are included, just search for bcp47 in translate.determineLocale.spec.js)
• please rename the resolver name: lang_only => iso639-1
• the new option should be mentioned in the list of available (incl. an example, i.e. same as in test?): $translateProvider.uniformLanguageTag

[knalli]

Superseeded by 1e037ec