Added \! (negative thin space)

Test Plan: Parses and renders locally. \!\, and \,\! are now no-ops.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D6395

Parser.js

@@ -289,6 +289,7 @@ var copyFuncs = {
         "\\nleq"
     ],
     "spacing": [
+        "\\!",
         "\\ ",
         "\\,",
         "\\:",

buildTree.js

@@ -287,7 +287,8 @@ var groupTypes = {
                 "\\quad": "quad",
                 "\\;": "thickspace",
                 "\\:": "mediumspace",
-                "\\,": "thinspace"
+                "\\,": "thinspace",
+                "\\!": "negativethinspace"
             };
 
             return makeSpan(["mord", "mspace", spacingClassMap[group.value]]);

static/katex.less

@@ -232,6 +232,10 @@ big parens
     .mspace {
         display: inline-block;
 
+        &.negativethinspace {
+            margin-left: -@thinspace;
+        }        
+
         &.thinspace {
             width: @thinspace;
         }