Supports Rename Glyphs parameter

Some issues are talking about it #498 #126 and this one aims to list all the useful parameters that could be handled from source:

- [ ] Remove glyphs
- [x] Keep glyphs
- [x] Rename glyphs
- [ ] Remove features
- [x] Replace feature
- [ ] Remove classes

We are trying to unroll an update of [cormorant](https://github.com/CatharsisFonts/Cormorant/tree/master/4.%20Glyphs%20Source%20Files) which is creating different families by swapping stylistic sets, it looks like that in sources:

I see how I could reproduce the remove glyphs and remove feature with fonttools subsetter, but not the Rename Glyphs thing though (although making additional post-export script doesn't help the designer to use fontmake). We made a little test case for this particular parameter in case you want to implement it 😬

[test-swap.zip](https://github.com/googlefonts/glyphsLib/files/7389249/test-swap.zip) Exported result should be a circle.

cc @emmamarichal

Start with the linked test-swap.zip and inspect how glyphsLib currently handles the checked Keep glyphs and Replace feature parameters. Trace the source-to-UFO or fontmake export path for parameter handling, then use the test case to verify that the exported result is a circle.

Written by the indexing model from the issue text.