[assets] Some improvements to the assets tool.
[assets] Some improvements to the assets tool.

  * Calculates and stores sha256 of every asset.
  * Allows specifying additional file extensions to scan.

R=dnj@chromium.org, 9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org

Bug:
Change-Id: Iababb4bced14633c7ed788a6f5cf433caa33f951
Reviewed-on: https://chromium-review.googlesource.com/832995
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go