vpxdec: rework default output parameters

vpxdec: rework default output parameters

This patch reworks the default behavior of the tool to output Y4M
instead of writing individual raw frames. The relevant controls are
now:
  --yv12, --i420  - These options change the output format to be
                    raw planar data. The output will be Y4M unless
                    one of these options is specified.

  --flipuv        - Swaps the chroma planes. Works with Y4M output.

  -o, --output    - Sets the output filename. Defaults to stdout if
                    not specified. Supports escape character
                    expansion for frame width (%w) height (%h) and
                    sequence number (%1..%9). The --prefix option
                    has been removed in favor of this escape
                    expansion.

Since the output defaults to stdout if -o is not specified, an
error will be thrown if stdout is not connected to a pipe. This
can be overridden by specifying '-o -'.

Change-Id: I94e42c57ca75721fdd57a6129e79bcdb2afe5d4d