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.
Removing an important space.
Updating test.
Making btest-include test names portable.
Merge remote-tracking branch 'origin/master' into topic/dnthayer/doc-improvements
Improve documentation of btest sphinx functionality

Mentioned that a ".btest" file extension is added to test files.
Mentioned that btest-rst-cmd and btest-rst-pipe check the exit code
of the specified command line argument.
Explained how the btest-rst-include and "btest-rst-cmd -r" commands
locate the specified file.
Added documentation of the "-n" option to btest-rst-cmd and btest-rst-include.