Support v4 json format

[jlabeit]

Background
With the latest major release of i18next a new json format v4 has been introduced. This format is currently not supported by this project because the underlying project i18next-scanner does not support it.

Proposed solution
It seems that there are two project doing more or less the same:

1. https://github.com/i18next/i18next-scanner
2. https://github.com/i18next/i18next-parser

There is a discussion to merge these two projects but it seems nothing will happen there soon (i18next/i18next-scanner#216).

Currently the scanner is being used, which seems to be less maintained and does not supprot the new V4 format.
I would propose to migrate this plugin to use the second project. Just this line would need to change: https://github.com/ph1p/i18next-scanner-webpack/blob/master/index.js#L118