vpxenc: initial implementation of multistream support

vpxenc: initial implementation of multistream support

Add the ability to specify multiple output streams on the command line.
Streams are delimited by --, and most parameters inherit from previous
streams.

In this implementation, resizing streams is still not supported. It
does not make use of the new multistream support in the encoder either.
Two pass support runs all streams independently, though it's
theoretically possible that we could combine firstpass runs in the
future. The logic required for this is too tricky to do as part of this
initial implementation. This is mostly an effort to get the parameter
passing and independent streams working from the application's
perspective, and a later commit will add the rescaling and
multiresolution support.

Change-Id: Ibf18c2355f54189fc91952c734c899e5c072b3e0