Consolidate Geocoder language options and i18n language detection.

[fschmenger]

The language specific geocoder providerOptions should be derived from the active language as detected by the util/Locale.js module. Currently geocoder has an app-conf setting may look like the following and is redundant to the global lang definition:

"providerOptions": {
    "key": "",
    "lang": "nl-NL",
    "countrycodes": "nl",
    "limit": 5
  }

This topic is a follow up task to #205 and was discussed in #233.
Quoted from #233:

This requires some sort of mapping between the supported languages and the geocoder language codes.
Where do we want to put the mapping (e.g. configure in the 'lang' property or somewhere specific in 'wgu-geocoder'? I didnt study geocoder code closely enough to know the implications, so any ideas welcome here.