MINOR: cli/activity: add a thread number argument to "show activity"

MINOR: cli/activity: add a thread number argument to "show activity"

The output of "show activity" can be so large that the output is visually
unreadable on a screen. Let's add an option to filter on the desired
column (actually the thread number), use "0" to report only the first
column (aggregated/sum/avg), and use "-1", the default, for the normal
detailed dump.