ipptool: add a "$basename" variable

[wifiprintguy]

The $filename variable provides the entire path specified by the -f option. It would be nice if there was a way to trim that to its "basename" component (last element in the path) for when the filename is used in job naming, such as:

ATTR name "job-name" "Some test $user $basename $job-id"

so that looking at the job log can be useful when testing.

Moved istopwg/ippsample#277 here as per guidance from @michaelrsweet.

[michaelrsweet]

[master fa34156] Add basename variable (Issue #44)