Implement nanoconfig support

Hi,

As discussed in nanomsg mailing list, it's better to support [nanoconfig](http://github.com/nanomsg/nanoconfig) directly by the nanomsg bindings instead of creating separate. The pull request adds just that.

The support of nanoconfig is optional, and is silently disabled if the library is not installed at build time (for the C bindings) or at runtime (for ctypes bindings).

The patch is submitted under MIT license.