Add setOption API

Fixes #271

src/xterm.js

@@ -357,6 +357,17 @@ Terminal.prototype.focus = function() {
   return this.textarea.focus();
 };
 
+/**
+ * Sets an option on the terminal.
+ */
+Terminal.prototype.setOption = function(key, value) {
+  if (!(key in Terminal.defaults)) {
+    throw new Error('No option with key "' + key + '"');
+  }
+  this[key] = value;
+  this.options[key] = value;
+};
+
 /**
  * Binds the desired focus behavior on a given terminal object.
  *