Change the approach to the directory-header control so we can set the
Argument value in the JSON response.

Stripping the trailing newline from the JSON output is annoying, but
looking over [1] I saw no easy way to add a newline to the JSON
output.  And with the general framework that commands/ attempts to be,
it feels a bit funny to customize the JSON output for a command-line
program.  Perhaps a workable solution is to have the command-line
client append newlines to any output that otherwise lacks them?  But
that seems like a change best left to a separate series.

[1]: http://golang.org/pkg/encoding/json/

License: MIT
Signed-off-by: W. Trevor King <wking@tremily.us>