Support Unicode ° (#1203)

Oddly, `unicode-math-table.txt` does not contain an entry for U+00B0. But `\degree` is surely the correct mapping.

test/katex-spec.js

@@ -2936,7 +2936,7 @@ describe("Unicode", function() {
     });
 
     it("should parse symbols", function() {
-        expect("£¥ðℂℍℑℓℕ℘ℙℚℜℝℤℲℵℶℷℸ⅁∀∁∂∃∇∞∠∡∢♠♡♢♣♭♮♯✓\u00b7").toParse();
+        expect("£¥ðℂℍℑℓℕ℘ℙℚℜℝℤℲℵℶℷℸ⅁∀∁∂∃∇∞∠∡∢♠♡♢♣♭♮♯✓°\u00b7").toParse();
     });
 
     it("should parse arrows", function() {