Add basic auto-render extension

Summary:
Add an auto-render extension to render math on a page. It
exposes a global function (maybe we should attach it to `katex`?) to
render math in an element. It comes with a README on how to use it.
Also, make `make build` build the minified file.

Fixes #26

Test Plan:
 - Visit http://localhost:7936/contrib/auto-render/
 - See that all of the math renders correctly
 - `make test`

Reviewers: alpert, kevinb

Reviewed By: kevinb

Differential Revision: https://phabricator.khanacademy.org/D16620

contrib/auto-render/splitAtDelimiters.js

@@ -0,0 +1,98 @@
+var findEndOfMath = function(delimiter, text, startIndex) {
+    // Adapted from
+    // https://github.com/Khan/perseus/blob/master/src/perseus-markdown.jsx
+    var index = startIndex;
+    var braceLevel = 0;
+
+    var delimLength = delimiter.length;
+
+    while (index < text.length) {
+        var character = text[index];
+
+        if (braceLevel <= 0 &&
+            text.slice(index, index + delimLength) === delimiter) {
+            return index;
+        } else if (character === "\\") {
+            index++;
+        } else if (character === "{") {
+            braceLevel++;
+        } else if (character === "}") {
+            braceLevel--;
+        }
+
+        index++;
+    }
+
+    return -1;
+};
+
+var splitAtDelimiters = function(startData, leftDelim, rightDelim, display) {
+    var finalData = [];
+
+    for (var i = 0; i < startData.length; i++) {
+        if (startData[i].type === "text") {
+            var text = startData[i].data;
+
+            var lookingForLeft = true;
+            var currIndex = 0;
+            var nextIndex;
+
+            nextIndex = text.indexOf(leftDelim);
+            if (nextIndex !== -1) {
+                currIndex = nextIndex;
+                finalData.push({
+                    type: "text",
+                    data: text.slice(0, currIndex)
+                });
+                lookingForLeft = false;
+            }
+
+            while (true) {
+                if (lookingForLeft) {
+                    nextIndex = text.indexOf(leftDelim, currIndex);
+                    if (nextIndex === -1) {
+                        break;
+                    }
+
+                    finalData.push({
+                        type: "text",
+                        data: text.slice(currIndex, nextIndex)
+                    });
+
+                    currIndex = nextIndex;
+                } else {
+                    nextIndex = findEndOfMath(
+                        rightDelim,
+                        text,
+                        currIndex + leftDelim.length);
+                    if (nextIndex === -1) {
+                        break;
+                    }
+
+                    finalData.push({
+                        type: "math",
+                        data: text.slice(
+                            currIndex + leftDelim.length,
+                            nextIndex),
+                        display: display
+                    });
+
+                    currIndex = nextIndex + rightDelim.length;
+                }
+
+                lookingForLeft = !lookingForLeft;
+            }
+
+            finalData.push({
+                type: "text",
+                data: text.slice(currIndex)
+            });
+        } else {
+            finalData.push(startData[i]);
+        }
+    }
+
+    return finalData;
+};
+
+module.exports = splitAtDelimiters;