Update documentation about __toString inconsistency, add test

Summary: Seems that we match php5 and only allow strings now. Found while investigating LeviM's questions for the return type RFC.

Reviewed By: @jdelong

Differential Revision: D1555431
Support mappy ini configuration

Summary: Along with `std::vector<std::string>`, `std::map<std::string, std::string>` is the
other most common collection-style ini configuration.

Support this construct.

Once these two basics are supported, then we tackle the non-stringy vectors and maps. That will be a little harder.

Reviewed By: @ptarjan

Differential Revision: D1516955