Add ability to export gasp tables

[m4rc1e]

Our hinters provide us with hinted ttfs which include hand tuned gasp tables. It would be nice to have an option which exports and merges this table.

@madig if this sounds like a nice feature, I'll happily submit a pr.

Should stop issues such as google/fonts#3876 from happening.

[madig]

Internally, we modify the UFO or Glyphs.app file gasp entry and then insert the hinting later in the pipeline. How do you do it?

[m4rc1e]

• Give hinter what we consider to be the final ttfs
• Hinter hints ttfs
• Hinter prs back ttfs
• We then use vttLib to export the hinting data to xml and delete the hinted ttfs since the source is now xml
• When we run our build chain, the xml is merged then compiled.

[m4rc1e]

We treat setting the gasp table as a job for the hinter.

[madig]

After a quick internal poll, I say: sure, send a PR, but make it 1) not dump the gasp info unless the user gives a --dump-gasp switch or something and 2) not change the gasp table if no gasp info exist. So that our use case of putting stuff into the UFO continues to work unchanged. Thanks!