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

src/buildHTML.js

@@ -1178,6 +1178,25 @@ groupTypes.rule = function(group, options, prev) {
     return rule;
 };
 
+groupTypes.kern = function(group, options, prev) {
+    // Make an empty span for the rule
+    var rule = makeSpan(["mord", "rule"], [], options.getColor());
+
+    var dimension = 0;
+    if (group.value.dimension) {
+        dimension = group.value.dimension.number;
+        if (group.value.dimension.unit === "ex") {
+            dimension *= fontMetrics.metrics.xHeight;
+        }
+    }
+
+    dimension /= options.style.sizeMultiplier;
+
+    rule.style.marginLeft = dimension + "em";
+
+    return rule;
+};
+
 groupTypes.accent = function(group, options, prev) {
     // Accents are handled in the TeXbook pg. 443, rule 12.
     var base = group.value.base;

src/buildMathML.js

@@ -430,6 +430,13 @@ groupTypes.rule = function(group) {
     return node;
 };
 
+groupTypes.kern = function(group) {
+    // TODO(kevin): Figure out if there's a way to add space in MathML
+    var node = new mathMLTree.MathNode("mrow");
+
+    return node;
+};
+
 groupTypes.llap = function(group, options) {
     var node = new mathMLTree.MathNode(
         "mpadded", [buildGroup(group.value.body, options)]);

src/functions.js

@@ -187,6 +187,16 @@ defineFunction("\\rule", {
     };
 });
 
+defineFunction("\\kern", {
+    numArgs: 1,
+    argTypes: ["size"],
+}, function(context, args) {
+    return {
+        type: "kern",
+        dimension: args[0].value,
+    };
+});
+
 // A KaTeX logo
 defineFunction("\\KaTeX", {
     numArgs: 0,