Added requirement for PyYAML.
Made a prototype yaml player.
Increased default bitrate of webm output.
Added -b and -o options to CLI.

-b produces an audible beep when encoding finishes.
-o allows specifying one or more output files.
Created a YAML validator and a "new" vid subcommand.

The YAML validator gives the user useful error messages.
The "new" subcommand prints a YAML template to stdout. It serves
both as a starting point for editing a movie and as user documentation.
Call YAMLReader instance from vid script. Corrected bugs in YAMLReader.