Improve reporting

[shazow]

• Added a bunch of TODOs for later
• Added histogram implementation with percentiles
• Revamped report output
• Switched to seconds for endpoint reporting, rather than duration-native units. Not sure if I'm happy with this.
• Percentile needs more testing, I worry there's an off-by-one error there.

Latest output:

Endpoints:

0. "https://shazow.net/"

   Requests:   25.51 per second
   Timing:     0.0392s avg, 0.0065s min, 1.0279s max

   Percentiles:
     25% in 0.0090s
     50% in 0.0100s
     75% in 0.0116s
     90% in 0.0152s
     95% in 0.3767s
     99% in 1.0279s

   Errors: 100.00%
     100 × "bad status code: 405"

** Summary for 1 endpoints:
   Completed:  100 results with 100 total requests
   Timing:     39.200052ms request avg, 1.413239503s total run time
   Errors:     100 (100.00%)
   Mismatched: 0