Add \underline support (#456)

![image](https://cloud.githubusercontent.com/assets/6820/14412284/e63218b6-ff13-11e5-912c-fc6e30928b93.png)

Fixes #209.

src/buildHTML.js

@@ -51,6 +51,7 @@ var groupToType = {
     op: "mop",
     katex: "mord",
     overline: "mord",
+    underline: "mord",
     rule: "mord",
     leftright: "minner",
     sqrt: "mord",
@@ -928,6 +929,32 @@ groupTypes.overline = function(group, options, prev) {
     return makeSpan(["overline", "mord"], [vlist], options.getColor());
 };
 
+groupTypes.underline = function(group, options, prev) {
+    // Underlines are handled in the TeXbook pg 443, Rule 10.
+
+    // Build the inner group.
+    var innerGroup = buildGroup(group.value.body, options);
+
+    var ruleWidth = fontMetrics.metrics.defaultRuleThickness /
+        options.style.sizeMultiplier;
+
+    // Create the line above the body
+    var line = makeSpan(
+        [options.style.reset(), Style.TEXT.cls(), "underline-line"]);
+    line.height = ruleWidth;
+    line.maxFontSize = 1.0;
+
+    // Generate the vlist, with the appropriate kerns
+    var vlist = buildCommon.makeVList([
+        {type: "kern", size: ruleWidth},
+        {type: "elem", elem: line},
+        {type: "kern", size: 3 * ruleWidth},
+        {type: "elem", elem: innerGroup},
+    ], "top", innerGroup.height, options);
+
+    return makeSpan(["underline", "mord"], [vlist], options.getColor());
+};
+
 groupTypes.sqrt = function(group, options, prev) {
     // Square roots are handled in the TeXbook pg. 443, Rule 11.
 

src/buildMathML.js

@@ -422,6 +422,20 @@ groupTypes.overline = function(group, options) {
     return node;
 };
 
+groupTypes.underline = function(group, options) {
+    var operator = new mathMLTree.MathNode(
+        "mo", [new mathMLTree.TextNode("\u203e")]);
+    operator.setAttribute("stretchy", "true");
+
+    var node = new mathMLTree.MathNode(
+        "munder",
+        [buildGroup(group.value.body, options),
+         operator]);
+    node.setAttribute("accentunder", "true");
+
+    return node;
+};
+
 groupTypes.rule = function(group) {
     // TODO(emily): Figure out if there's an actual way to draw black boxes
     // in MathML.

src/functions.js

@@ -170,6 +170,17 @@ defineFunction("\\overline", {
     };
 });
 
+// An underline
+defineFunction("\\underline", {
+    numArgs: 1,
+}, function(context, args) {
+    var body = args[0];
+    return {
+        type: "underline",
+        body: body,
+    };
+});
+
 // A box of the width and height
 defineFunction("\\rule", {
     numArgs: 2,