Adds skiaperf.py for formatting skpbench.py outputs for Skia Perf.
Also renames parseskpbench.py to sheet.py.
BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4657
Change-Id: I758678e1c589b15ec2d07c43e4921663e919b47b
Reviewed-on: https://skia-review.googlesource.com/4657
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
