[Feature request] Support libconfig

[Philipp-M]

libconfig seems to be similar to the other configuration languages.
There is a python lib (jsonlibconfig), which supports json <-> libconfig conversion already, which might be considered to support this feature request.

[dbohdan]

I am not against support for libconfig, but I won't implement it myself. I'll merge a PR with

• High-quality code consistent with the code for the already present formats;
• Similar tests.

Look at #23 for how a contributor did it for CBOR.