Add better comment parsing to hooke.ui.commandline.

Add better comment parsing to hooke.ui.commandline.

The old implemtation only handled comments after valid function calls:
  hooke> load_playlist test # a comment
but failed on pure comments and in help
  hooke> # failing comment
  hooke> help # failing comment
The new implemation handles all of these.

I changed empty lines to no-ops (rather than repeating the last
commant), because explicit is better than implicit.