Support Unicode Symbols (#950)

* Support Unicode Symbols

This is the third in a series of PRs to give KaTeX the ability to recognize Unicode character input. This one supports some but not all letter-like symbols and some other symbols. All the characters in this PR will produce `textord` atoms in `math` mode.

The list is not comprehensive. It’s a bit eclectic. Some of these characters are important. Others were just easy to include. A few important characters are omitted because it wasn’t clear to me which is the best way to implement them. I’ll address those characters in future PRs.

* Add comments. Remove RegEx.

* Fix omitted parens

test/katex-spec.js

@@ -2678,6 +2678,10 @@ describe("Unicode", function() {
         expect("⊂⊃⊆⊇⊏⊐⊑⊒⊢⊣⊩⊪⊸⋈⋍⋐⋑⋔⋙⋛⋞⋟⌢⌣⩾⪆⪌⪕⪖⪯⪰⪷⪸⫅⫆").toParse();
     });
 
+    it("should parse symbols", function() {
+        expect("£¥ðℂℍℑℓℕ℘ℙℚℜℝℤℲℵℶℷℸ⅁∀∁∂∃∇∞∠∡∢♠♡♢♣♭♮♯✓").toParse();
+    });
+
     it("should parse arrows", function() {
         expect("←↑→↓↔↕↖↗↘↙↚↛↞↠↢↣↦↩↪↫↬↭↮↰↱↶↷↼↽↾↾↿⇀⇁⇂⇃⇄⇆⇇⇈⇉").toParse();
     });