Add support for \kern

Summary:
This only supports em and ex units and doesn't handle vertical layouts.
Negative kerning works.

Test Plan:
- make test
- make screenshots (verify that d is slightly overlapping c in the screenshots)

Reviewers: emily

test/katex-spec.js

@@ -909,6 +909,31 @@ describe("A rule parser", function() {
     });
 });
 
+describe("A kern parser", function() {
+    var emKern = "\\kern{1em}";
+    var exKern = "\\kern{1ex}";
+    var badUnitRule = "\\kern{1px}";
+    var noNumberRule = "\\kern{em}";
+
+    it("should list the correct units", function() {
+        var emParse = getParsed(emKern)[0];
+        var exParse = getParsed(exKern)[0];
+
+        expect(emParse.value.dimension.unit).toEqual("em");
+        expect(exParse.value.dimension.unit).toEqual("ex");
+    });
+
+    it("should not parse invalid units", function() {
+        expect(badUnitRule).toNotParse();
+        expect(noNumberRule).toNotParse();
+    });
+
+    it("should parse negative sizes", function() {
+        var parse = getParsed("\\kern{-1em}")[0];
+        expect(parse.value.dimension.number).toBeCloseTo(-1);
+    });
+});
+
 describe("A left/right parser", function() {
     var normalLeftRight = "\\left( \\dfrac{x}{y} \\right)";
     var emptyRight = "\\left( \\dfrac{x}{y} \\right.";

test/screenshotter/ss_data.yaml

@@ -58,6 +58,7 @@ FractionTest: \dfrac{a}{b}\frac{a}{b}\tfrac{a}{b}\;-\dfrac12\;1\tfrac12
 Functions: \sin\cos\tan\ln\log
 GreekLetters: \alpha\beta\gamma\omega
 KaTeX: \KaTeX
+Kern: \frac{a\kern{1em}b}{c}a\kern{1em}b\kern{1ex}c\kern{-0.25em}d
 Lap: ab\llap{f}cd\rlap{g}h
 LeftRight: \left( x^2 \right) \left\{ x^{x^{x^{x^x}}} \right.
 LeftRightListStyling: a+\left(x+y\right)-x