[qa] Allow multiple --extra-fontbakery-args

[simoncozens]

--extra-fontbakery-args is a flag on gftools-qa which passes another flag to fontbakery. Of course this causes problems. If you say --extra-fontbakery-args --foo this gets interpreted as a --foo flag to gftools-qa. "--foo" doesn't help because the shell eats the quotes. Apparently the right way to do it is --extra-fontbakery-args="--foo" which parses properly. But then --extra-fontbakery-args="--foo bar" works on the gftools-qa side, but adds a single string "--foo bar" string to fontbakery's argv, which does not parse on that side. Much joy. And you really don't want to get into splitting arguments on spaces and feeding them to argv, because --html "A Report.html" should be two arguments, not three. And --extra-fontbakery-args="--foo" "bar" doesn't work either.

So (deep breath) it turns out the right way to do it is --extra-fontbakery-args="--foo" --extra-fontbakery-args="bar", and that in turn means we need to use action="append" instead of nargs="*".

[m4rc1e]

Thank you!