MEDIUM: stats: Add JSON output option to show (info|stat)

MEDIUM: stats: Add JSON output option to show (info|stat)

Add a json parameter to show (info|stat) which will output information
in JSON format. A follow-up patch will add a JSON schema which describes
the format of the JSON output of these commands.

The JSON output is without any extra whitespace in order to reduce the
volume of output. For human consumption passing the output through a
pretty printer may be helpful.

e.g.:
$ echo "show info json" | socat /var/run/haproxy.stat stdio | \
     python -m json.tool

STAT_STARTED has bee added in order to track if show output has begun or
not. This is used in order to allow the JSON output routines to only insert
a "," between elements when needed. I would value any feedback on how this
might be done better.

Signed-off-by: Simon Horman <horms@verge.net.au>