benchmark: add output format option [csv]

This commit adds an `OUTPUT_FORMAT` environment variable option for
all benchmark tests that allow either 'csv' or 'default' output. Default
output has been left unchanged, and csv output prints out the csv
headers along with the csv formatted per-test output, each test also
seperated by a newline.

It can be used like the following:
$ OUTPUT_FORMAT=csv iojs benchmark/common.js http

Not specifying the OUTPUT_FORMAT env var will default to 'default'.
Specifying a bad value will throw an error.

PR-URL: https://github.com/iojs/io.js/pull/777


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>