fix(index): escape invalid characters (#43)

* Handle special whitespace characters.

If your raw text has a string with Line separator or Paragraph separator,
they need to be handled outside of JSON.stringify:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify#Issue_with_plain_JSON.stringify_for_use_as_JavaScript

The fix here is to replace those characters as directed in the MDN article above.

I also changed the formatting from tabs to 2 spaces :-p

* Address code review.

* Rename some variables.

* Remove cacheable.

Turns out cacheable is only used in webpack 1.
So we are removing it.

index.js

@@ -1,9 +1,13 @@
 /*
-	MIT License http://www.opensource.org/licenses/mit-license.php
-	Author Tobias Koppers @sokra
+  MIT License http://www.opensource.org/licenses/mit-license.php
+  Author Tobias Koppers @sokra
 */
-module.exports = function(content) {
-	this.cacheable && this.cacheable();
-	this.value = content;
-	return "module.exports = " + JSON.stringify(content);
+module.exports = function(source) {
+  this.value = source;
+
+  var json = JSON.stringify(source)
+    .replace(/\u2028/g, '\\u2028')
+    .replace(/\u2029/g, '\\u2029');
+
+  return "module.exports = " + json;
 }