option to drop kern table when GPOS kern is available

Hello,

I added a new option called `--legacy-kern` and set the default to False (i.e. equivalent to `--no-legacy-kern`).
I simply look for "kern" in the available GPOS features, and if I find one, then I drop the "kern" table.
When `--legacy-kern` is set to True, the "kern" table is not dropped.
Does "legacy-kern" make sense for you as a name for the option?

All best,

Cosimo