feature request: incremental filter assembling

[zmike]

In Mesa CI, we run VVL with khronos_validation.message_id_filter = <a billion exceptions>. This has to be on a single line, which is annoying since it both scrolls way offscreen and doesn't allow for documentation.

I'd like to see this improved so I could do something like

# comment explaining
khronos_validation.message_id_filter =
# comment explaining
khronos_validation.message_id_filter += VUID1
# comment explaining
khronos_validation.message_id_filter += VUID2
# comment explaining
khronos_validation.message_id_filter += VUID3