perf/vg_perf --outer-args: either replace the predefined outer args, or append to it.
Currently, vg_perf predefines a set of standard outer tool args according to the tool.
These predefined args can be replaced by another set using "--outer-args=xxx yyy zzzz".
But often, we want to add (or override) only a few args.
So, modify vg_perf so that if the first letter of --outer-args is a + character, then
the provided args are appended to the predefined args