Added 14 Symbols

Hello Emily and Ben,

Apologies for not emailing you back Emily. After reading the contribution file, we decided to add a few symbols. If the pull request is approved we get a bonus 5 lab marks. It's not worth a lot but anything helps. Hopefully we helped KaTeX a little bit.

The symbols added are: ⊎ ⊓ ⊔ ⊖ ◯ ‡ ⨿ ∃ ∄ ∀ ⇒ ⇔ ∍