Add getFormat(text) - returns format (string) of given text

It's purposively more inclusive than answer-types' predicate.forms, but more for sake of brevity than necessity. Optimally, I could just tap into answer-types' predicate.forms, go through each one in some order and return the key if it matches. It might also be wise to do it the other way, have several function in KhanUtil for isMixed(text), isInteger(text), isDecimal(text) accounting for commas/spaces/badLeadingZeros, etc. and then having those functions called from inside answer-types.

If you have strong feelings against this, lemme know

Test Plan: Try "1", "1/2", "3/2", "1 1/2", "1 3/2", "1.3", "1,3", "3 1,3.4" (deliberate, if not foolish), "pi", "topic", "topau", "topau%"

Auditors: eater, jack