Implement \verb (#614)

* Implement \verb

* Implement @gagern's comments

* \verb: look up characters one at a time.

* Add screenshot test for \verb

* Add error tests for \verb

* Include space symbol in typewriter font, and fix single quotes

This is based on https://github.com/Khan/MathJax-dev/pull/2
which hasn't been accepted yet at the time this commit is made.

* Add \verb* tests

* \verb should use Typewriter-Regular font!

* Switch \verb to use text mode and no-break space.

* Screenshot update with Typewriter-Regular

* \verb test: fix *, add commas to make spaces clear

* Fix spaces and style handling

* Implement @kevinbarabash's comments

* Make error clearly an assertion failure

* verb screenshot for Chrome

src/buildMathML.js

@@ -455,6 +455,13 @@ groupTypes.sizing = function(group, options) {
     return node;
 };
 
+groupTypes.verb = function(group, options) {
+    const text = new mathMLTree.TextNode(buildCommon.makeVerb(group, options));
+    const node = new mathMLTree.MathNode("mtext", [text]);
+    node.setAttribute("mathvariant", fontMap["mathtt"].variant);
+    return node;
+};
+
 groupTypes.overline = function(group, options) {
     const operator = new mathMLTree.MathNode(
         "mo", [new mathMLTree.TextNode("\u203e")]);