Add support for gotext format

[mikhirev]

Please consider adding a support of gotext translation format. It is a golang's native translation format based on JSON. Unfortunately it is poorly documented.

References:

• An intro article, not comprehensive, just to make an idea.
• golang.org/x/text/message/pipeline package documentation. It provides data structures serialized to JSON.
• golang.org/x/text/feature/plural package documentation, contains description of how messages are pluralized.