Add a JSON parser

This PR adds a full-featured JSON parser, according to the [ECMA-404 spec](http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-404.pdf).  The parser is however intentionally very forgiving on broken or invalid input (it especially is able to skip invalid values and get back on its feet for the next one).

I am however not certain whether we want a JSON parser in CTags directly: JSON is not a programming language and extracting tags from it does not seem to make much sense for indexing purposes.  However, tools that use CTags to provide in-file navigation would benefit from it, as it would basically create a "table of contents".
Similarly, Geany has a parser for INI-style configuration files that could easily be imported if it's something we want in CTags (I'm not sure where it comes from, but it possibly is genuine as it doesn't seem to exist in the upstreams I know)

Opinions?