Add customizable indentation and unindent

You can now press shift+tab to unindent.

src/codeflask.js

@@ -1,5 +1,5 @@
-function CodeFlask() {
-
+function CodeFlask(indent) {
+  this.indent = indent || "    ";
 }
 
 CodeFlask.prototype.run = function(selector, opts) {
@@ -109,11 +109,21 @@ CodeFlask.prototype.handleInput = function(textarea, highlightCode, highlightPre
         inputVal = input.value;
 
         // If TAB pressed, insert four spaces
-        if(e.keyCode === 9){
-            input.value = inputVal.substring(0, selStartPos) + "    " + inputVal.substring(selStartPos, input.value.length);
-            input.selectionStart = selStartPos + 4;
-            input.selectionEnd = selStartPos + 4;
-            e.preventDefault();
+        if (e.keyCode === 9) {
+          e.preventDefault();
+
+          // Allow shift-tab if the 4 characters in front are spaces
+          if (e.shiftKey && inputVal.substring(selStartPos - self.indent.length, selStartPos) == self.indent) {
+            input.value = inputVal.substring(0, selStartPos - self.indent.length) +
+                          inputVal.substring(selStartPos, input.value.length);
+            input.selectionStart = selStartPos - self.indent.length;
+            input.selectionEnd = selStartPos - self.indent.length;
+          } else {
+            input.value = inputVal.substring(0, selStartPos) + self.indent +
+                          inputVal.substring(selStartPos, input.value.length);
+            input.selectionStart = selStartPos + self.indent.length;
+            input.selectionEnd = selStartPos + self.indent.length;
+          }
 
             highlightCode.innerHTML = input.value.replace(/&/g, "&")
                 .replace(/</g, "&lt;")