Use fontTools config

[belluzj]

Based on Simon's refactoring of argument passing in #533

The goal is to use the fontTools config module that is being developed in fonttools/fonttools#2416

I would like to implement 2 things in ufo2ft:

• make sure to disable GPOS optimization while building individual masters that will later be merged into a VF.
• pass-through config options from ufo2ft's entrypoints to fontTools TTFont() (however, in Add a facility to pass options to various fontTools modules fonttools/fonttools#2349 Behdad was saying that ufo2ft and fontmake should not have to pass these options around, so I need to figure that out)

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[belluzj]

Cosimo: call this fontToolsConfig

[anthrotype]

Closing in favor of #607