Adds `use_framing` config to all output plugins, and `Encode` and
`UsesFraming` methods to the OutputRunner. If use_framing is true,
then OutputRunner.Encode will prepend the framing, and UsesFraming
will return true. If false, OutputRunner.Encode will return the output
from Encoder.Encode unadorned, and UsesFraming will return false.

All outputs currently using encoders for serialization have been
updated to use the new spelling.
Merge pull request #931 from mozilla-services/outputrunner-encodes

OutputRunner encoding