Fix the greediness of the \color function

Summary:
The greediness of the `\color` function wasn't set correctly,
leading to expressions like `\color{red}\text{a}` parsing correctly,
when they shouldn't. (This is based on how MathJax parses, since TeX
doesn't have a `\color` function, so MathJax is the standard).

Test Plan:
 - Make test
 - See that `\color{red}\text{a}` doesn't parse (like MathJax)
 - See that `\color{red}{\text{a}}` does parse (like MathJax)
 - See that `\color{red}\frac12` doesn't parse (like MathJax)
 - See that `\color{red}{\frac12}` does parse (like MathJax)
 - See that `\red\text{a}` doesn't parse (like MathJax)
 - See that `\red{\text{a}}` does parse (like MathJax)
 - See that `\red\frac12` doesn't parse (like MathJax)
 - See that `\red{\frac12}` does parse (like MathJax)

Reviewers: alpert

Reviewed By: alpert

Differential Revision: https://phabricator.khanacademy.org/D17130

src/functions.js

@@ -112,6 +112,7 @@ var functions = {
     "\\color": {
         numArgs: 2,
         allowedInText: true,
+        greediness: 3,
         argTypes: ["color", "original"],
         handler: function(func, color, body) {
             // Normalize the different kinds of bodies (see \text above)
@@ -235,6 +236,7 @@ var duplicatedFunctions = [
         data: {
             numArgs: 1,
             allowedInText: true,
+            greediness: 3,
             handler: function(func, body) {
                 var atoms;
                 if (body.type === "ordgroup") {

test/katex-spec.js

@@ -676,6 +676,13 @@ describe("A color parser", function() {
     it("should not parse a bad custom color", function() {
         expect(badCustomColorExpression).toNotParse();
     });
+
+    it("should have correct greediness", function() {
+        expect("\\color{red}a").toParse();
+        expect("\\color{red}{\\text{a}}").toParse();
+        expect("\\color{red}\\text{a}").toNotParse();
+        expect("\\color{red}\\frac12").toNotParse();
+    });
 });
 
 describe("A tie parser", function() {