Added various set theory symbols

\subset, \supset, \subseteq, \supseteq, \cap, \cup, \setminus

I would assume \cap, \cup and \setminus belong in "op", but changing rel to op resulted in the following error when using them:

    TypeError: 'undefined' is not an object (evaluating 'group.value.body.length')  katex.js:1934

Bug, or me doing something wrong?

Also, I was unsure of how you want to do negation of symbols like these. Usually both \nsubset and \not\subset works, and I think some of the resulting symbols are combinations, not separate symbols?