Add a \color command for custom colors

Summary:
Keep track of the color inside the style now, and use that when we are
rendering things. Added a custom lexing mode to handle lexing colors correctly.
Prefixed the old katex colors (internally) with "katex-" to distinguish them
from CSS colors.

Test Plan:
 - Run the normal tests, see they work
 - Run the huxley tests, see that they didn't change except for the color one
   which looks right

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D7763

test/huxley/Huxleyfile

@@ -11,7 +11,7 @@ url=http://localhost:7936/test/huxley/test.html?m=\dfrac{\frac{a}{b}}{\frac{c}{d
 url=http://localhost:7936/test/huxley/test.html?m=a^{a^a_a}_{a^a_a}
 
 [Colors]
-url=http://localhost:7936/test/huxley/test.html?m=\blue{a}\green{b}\red{c}
+url=http://localhost:7936/test/huxley/test.html?m=\blue{a}\color{%%230f0}{b}\color{red}{c}
 
 [GreekLetters]
 url=http://localhost:7936/test/huxley/test.html?m=\alpha\beta\gamma\omega

test/katex-tests.js

@@ -496,3 +496,41 @@ describe("A text parser", function() {
         expect(group[3].type).toMatch("spacing");
     });
 });
+
+describe("A color parser", function() {
+    var colorExpression = "\\blue{x}";
+    var customColorExpression = "\\color{#fA6}{x}";
+    var badCustomColorExpression = "\\color{bad-color}{x}";
+
+    it("should not fail", function() {
+        expect(function() {
+            parseTree(colorExpression);
+        }).not.toThrow();
+    });
+
+    it("should build a color node", function() {
+        var parse = parseTree(colorExpression)[0];
+
+        expect(parse.type).toMatch("color");
+        expect(parse.value.color).toBeDefined();
+        expect(parse.value.value).toBeDefined();
+    });
+
+    it("should parse a custom color", function() {
+        expect(function() {
+            parseTree(customColorExpression);
+        }).not.toThrow();
+    });
+
+    it("should correctly extract the custom color", function() {
+        var parse = parseTree(customColorExpression)[0];
+
+        expect(parse.value.color).toMatch("#fA6");
+    });
+
+    it("should not parse a bad custom color", function() {
+        expect(function() {
+            parseTree(badCustomColorExpression);
+        }).toThrow();
+    });
+});