main: add -filter option to filter on process name

Filter accepts a regular expression that, when given, will be used to
filter processes on which to report.

I thought about adding a flag where you could specify PIDs to watch,
but that seemed more brittle - processes get restarted more often than
you would like.  By filtering on process name, its easier to use psm
in long running contexts, like cronjobs.

The result ends up being:

(master) [bpowers@fina psm]$ psm -filter=chrome
    MB RAM    SHARED   SWAPPED	PROCESS (COUNT)
       0.1       0.0          	chrome-sandbox (1)
     367.6      52.7          	chrome (11)

Where the total reported is the sum of the processes that match the
filter regex.
doc: describe -filter in the manpage