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

test/screenshotter/ss_data.yaml

@@ -279,6 +279,13 @@ UnsupportedCmds:
     noThrow: 1
     errorColor: "#dd4c4c"
     nolatex: deliberately does not compile
+Verb: |
+    \begin{array}{ll}
+      \verb \verb ,   & \verb|\verb  |, \\
+      \verb* \verb* , & \verb*|\verb* |, \\
+      \verb!<x> & </y>! & \scriptstyle\verb|ss verb| \\
+      \verb*!<x> & </y>! & \small\verb|sm verb| \\
+    \end{array}
 VerticalSpacing:
     pre: potato<br>blah
     tex: x^{\Huge y}z