Support JsonPath for list querying

[renkun-ken]

jeroen/jsonlite#112

The example provided by @timelyportfolio should be very helpful.

[renkun-ken]

@timelyportfolio, I implement a list.query to query list object or direct json string using JsonPath.

[timelyportfolio]

Oh nice, I'll play with it. I think this does something that is basically impossible with R lists, and something I wanted in list.search. Do you have any immediate use cases?

I'll also try to evaluate some of the other packages. I like defiant.js a lot, but it is tricky in V8 since it requires the domparser of a browser and other browser things. It's nice in that it uses real XPath.

[renkun-ken]

After some consideration, I think jsonpath does nothing more than the original functions but some recursive node searching like what .. does in jsonpath. list.search should be enhanced so that more metadata are available.