ok, most features can be vias

This refactors most features out of ok's core into vias:
   -w    --> a .png dumping via, "png", opening the door to other types
   -m/-s --> a filtering via "filter"

Everything now can print a brief help message too.

Change-Id: I9e653aab98fd57182a6d458c7a80052130980284
Reviewed-on: https://skia-review.googlesource.com/10509
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
6 files changed