Allow only certain unicode symbols

[wclr]

How about adding an option to support allowing/forcing only user-defined symbols?

For example only forall - ∀.

[natefaubion]

I could see something like:

--unicode-arrows
--unicode-forall
--unicode-type

Where --unicode-always implies all three. What do you think?

[wclr]

I think it is how it should be. How this option will be set in rc config?

{
  "unicode": ["arrows", "forall"] 
}

?