Support Unicode \mid (#993)

Map ∣, U+2223, to `\mid`

test/katex-spec.js

@@ -2694,7 +2694,7 @@ describe("Unicode", function() {
     });
 
     it("should parse relations", function() {
-        expect("∈∋∝∼∽≂≃≅≈≊≍≎≏≐≑≒≓≖≗≜≡≤≥≦≧≫≬≳≷≺≻≼≽≾≿∴∵").toParse();
+        expect("∈∋∝∼∽≂≃≅≈≊≍≎≏≐≑≒≓≖≗≜≡≤≥≦≧≫≬≳≷≺≻≼≽≾≿∴∵∣").toParse();
     });
 
     it("should parse big operators", function() {