Support Unicode Arrows (#947)

* Support Unicode Arrows

This is the second in a series of PRs to give KaTeX the ability to recognize Unicode character input.

All the characters in this PR will produce `rel` atoms.

test/katex-spec.js

@@ -2638,6 +2638,14 @@ describe("Unicode", function() {
         expect("⊂⊃⊆⊇⊏⊐⊑⊒⊢⊣⊩⊪⊸⋈⋍⋐⋑⋔⋙⋛⋞⋟⌢⌣⩾⪆⪌⪕⪖⪯⪰⪷⪸⫅⫆").toParse();
     });
 
+    it("should parse arrows", function() {
+        expect("←↑→↓↔↕↖↗↘↙↚↛↞↠↢↣↦↩↪↫↬↭↮↰↱↶↷↼↽↾↾↿⇀⇁⇂⇃⇄⇆⇇⇈⇉").toParse();
+    });
+
+    it("should parse more arrows", function() {
+        expect("⇊⇋⇌⇍⇎⇏⇐⇑⇒⇓⇔⇕⇚⇛⇝⟵⟶⟷⟸⟹⟺⟼").toParse();
+    });
+
     it("should parse binary operators", function() {
         expect("±×÷∓∔∧∨∩∪≀⊎⊓⊔⊕⊖⊗⊘⊙⊚⊛⊝⊞⊟⊠⊡⊺⊻⊼⋇⋉⋊⋋⋌⋎⋏⋒⋓⩞").toParse();
     });