Script that imports glyphs from one font to another and takes GID list as input

I need a script to wrap up Open Sans after instantiation. `varLib` instantiates nicely (better than `mutatorMath` IMO) but composite hint metrics don't carry over (no surprise). I am looking for a script to handle this. Ideally, the script:

1. takes in a list of GIDs (to account for unencoded glyphs)
2. uses that list as a source to pull glyphs from one file and import them into another at the same GID (i.e. overwriting)
3. sends it to the VTTshell.exe Windows command line prompt to recompile

The first step is to autohint copies of the files, thereby fixing the composites. That can't be scripted, sadly. The script would then import those composites over the flawed glyphs in the source file and recompile using the VTT tool (`VTTShell.exe -a font.ttf`). Make sense?

Ugly accent example:

[cc @davelab6 & @felipesanches]