Add filter mode

This adds a shortcut (M-!) and a line edit derived filter mode
that runs a command as a subprocess and puts the output into a
new buffer.
Add key for filter mode

I forgot to add the shortcut in the previous commit
Filter command used for buffer name

The command line used for the filter is now passed through
godit.buffer_name to generate a unique name for the buffer
containing the filter results.
Add kill buffer and view with C-x K