Add string parsing (builtin, test and doc).

Well, I had to process such a log file... where a value from a JSON object was a query to our server.

...
{"date":"2014/03/05 15:25:42","ip":"81.83.144.50","query":"version=2014_02_20&language=nl&UUID=BE7FE9D9-83E1-4117-9ECF-69AEA4FF4124&dev=iPad3%2C3&os=7.0.6"}
{"date":"2014/03/05 15:27:43","ip":"91.183.33.9","query":"version=2014_02_20&language=fr&UUID=494018D1-6CAF-4D99-906B-088A9F9DCD02&dev=iPhone3%2C1&OS=7.0.6"}
...

Adding "parse" (somehow related to "split") made my processing more "natural"...  I hope it helps.  Thanks for creating jq.