helpers.sh: Add vgsys

vgsys - filters vgstash's output by system. Optionally allows one to
        specify the filter name to pass to vgstash. The default filter
        is 'allgames'.

For example, to show every game needed to be beaten on Steam, you'd use
something like this:

    vgsys backlog Steam

This is a handy feature to have when you want to narrow down your
options for what-to-play.

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