Feature request: Add support for new specialPGEnvironmentVars settings to support Unicode alternatives

[taniwallach]

I think the render should probably be able to handle the new control settings be added in: openwebwork/webwork2#1174 :

• $pg{specialPGEnvironmentVars}{parseAlternatives}
• $pg{specialPGEnvironmentVars}{convertFullWidthCharacters}
  The settings and the changes to PG are intended to support the use of some Unicode characters are equivalents for "standard" ones, which will be optionally handled by PG in PG 2.16 (PR to develop at present).

See:

• Attempt to support some standard math operators when included in answers using Unicode characters openwebwork/pg#517
• Add aliases to all context classes, and allow alternative tokens openwebwork/pg#518