Tweaks to Sphinx commands and directives.

btest-rst-*:
    - Always show line numbers.

    - Highlight the command executed.

    - rst-cmd-include gets an option -n <i> to include only upto i lines.

    - rst-cmd-include prefixes output with "<file>" to show what we're
      including.

btest-include:
    - Set Pygments language automatically if we show a file with an
      extension we know (in particular ".bro").

    - Prefix output with "<file>" to show what we're including. (TODO:
      This should be highlighted but that doesn't work currently for
      some reason).

    - TODO: This also should show lineno always but it likewise
      doesn't work yet for some reason.

Still not quite happy with the looks of the included examples, but
will fix later.