Version 0.2.0

• Add a binary for running metrics on y4m files (and other future formats)
  • This binary can be installed from the av-metrics-tool crate.
• Breaking change: Decoder<T> is changed to Decoder and has a new method,
  get_bit_depth added. This allows us to dynamically dispatch to the correct
  version of a metric based on the bit depth, without the compiler getting
  in our way.
• Add a workspace for managing the library and binary independently.