Support Unicode relations (#933)

* Support Unicode relations

This is the first in a series of PRs to give KaTeX the ability to recognize Unicode character input. The code in this PR follows the style of PR #410.
All the characters in this PR will produce rel atoms. I’ll submit PRs for other atom types later.

* Fix lint error.

* Correct mapping errors

This commit fixes a brain cramp of mine.

test/katex-spec.js

@@ -2616,6 +2616,14 @@ describe("Unicode", function() {
     it("should parse 'ΓΔΘΞΠΣΦΨΩ'", function() {
         expect("ΓΔΘΞΠΣΦΨΩ").toParse();
     });
+
+    it("should parse relations", function() {
+        expect("∈∋∝∼∽≂≃≅≈≊≍≎≏≐≑≒≓≖≗≜≡≤≥≦≧≫≬≳≷≺≻≼≽≾≿").toParse();
+    });
+
+    it("should parse more relations", function() {
+        expect("⊂⊃⊆⊇⊏⊐⊑⊒⊢⊣⊩⊪⊸⋈⋍⋐⋑⋔⋙⋛⋞⋟⌢⌣⩾⪆⪌⪕⪖⪯⪰⪷⪸⫅⫆").toParse();
+    });
 });
 
 describe("The maxSize setting", function() {