Enable re-printing of results without re-building or re-running

Summary: Many times I forget to add the --remarkup flag to print the
results in a phabricator-friendly fashion. This diff adds a way to get
benchy to re-print previous results without re-building or re-running
the benchmarks.

Reviewed By: @jdelong

Differential Revision: D1647396
Add JSON output support to benchy

Summary: This will allow benchy to communicate with other future services more
easily.

Reviewed By: @jdelong

Differential Revision: D1646887

Signature: t1:1646887:1414608128:32c5ca67a0a2215a7c39efdb26126668437903d6