Add parameters to adjust JSON contents

By default all result and request data is enabled in the JSON result. When doing large numbers of runs on pages with lots of resources this can lead to JSON results which are very large (5MB+) which slow down bulk processing and require large amounts of RAM to parse.

These URL parameters allow the JSON result to be modified and the desired metrics selected:

runs=0 // disable runs
average=0 // disable average
standard=0 // disable standard deviation
median=0 // disable median
requests=0 // disable request details