Improved menuing user experience.

Added unknown command message to appear after unknown command entry.

Example:

    Red Example Score (2013)

       => __init__.py
       => __metadata__.py
       => build/
       => distribution/
       => makers/
       => materials/
       => segments/
       => stylesheets/
       => test/

          build (u)
          distribution (d)
          makers (k)
          materials (m)
          segments (g)
          stylesheets (y)

    > asdf
    Unknown command: 'asdf'.

    > q