KaTeX: Don't generate an exception when an arbitrary symbol is used inside a formula

Hi Kevin and thanks for your work first;

current behaviour of KaTeX is such that most of the symols aside latin letters, numbers and special symbols cause ParseError. Couple of examples:

1. a formula with dimensions of quantities, in say, Russian, like {1 Н\over 2 м^2} = 0,5 Па (in English that would be: {1 N\over m^2} = 0.5 Pa)
2. a formula with an “already parsed” symbol like A = ρgh (this would be nice for TiddlyWiki as the editable content is this case is more readable)

It would be nice if those symbols are displayed “as is” (except for the font size). In fact, #379 is about this, too.