option to dump trace in structured format

[rustyrazorblade]

I love having --features peg/trace, but I found it was difficult to read through, so i've wrapped the output with a little script that handles indenting to make it easier to understand.

Then i started thinking, it would be event better if I visualized the parser output in one of a couple ways - either a collapsable tree or simply through something like graphviz.

To make this process somewhat straightforward, I'd like to add support for dumping the peg trace as structured JSON to a file. Thoughts?

[kevinmehall]

Related: @fasterthanlime's pegviz is simply parsing the existing messages to generate a nice HTML visualization.