Revert "Add :format and :keep_original options to :asset_hash. Closes #1257"

This reverts commit 1f98d0f4f01cde226df3e1f06d25b839fc34d753.

Conflicts:
	middleman-core/lib/middleman-core/extensions/asset_hash.rb
Merge pull request #1280 from bhollis/revert-asset-hash

Revert 1f98d0f