Config: Add "pre-release" option

JQuery UI began to publish pre-releases, eg. 1.11.0-beta.1, which can't be
described as stable nor legacy. So, it turned out that one more option was
needed: "pre-release".

Ref: #208

Author: rxaviers

config.json

@@ -2,7 +2,8 @@
 	"jqueryUi": [
 		{
 			"version": "1.11.0-beta.1",
-			"dependsOn": "jQuery1.6+"
+			"dependsOn": "jQuery1.6+",
+			"pre-release": true
 		},
 		{
 			"version": "1.10.4",

lib/config.js

@@ -8,6 +8,11 @@ Config = module.exports = function() {
 	// Validate jqueryUi, eg:
 	//   "jqueryUi": [
 	//     {
+	//       "version": "1.11.0-beta.1",
+	//       "dependsOn": "jQuery 1.7+",
+	//       "pre-release": true
+	//     },
+	//     {
 	//       "version": "1.10.0",
 	//       "dependsOn": "jQuery 1.7+",
 	//       "stable": true

lib/jquery-ui.js

@@ -38,7 +38,7 @@ function JqueryUi( path, options ) {
 	options = options || {};
 	this.dependsOn = options.dependsOn;
 	this.stable = options.stable;
-	this.label = this.stable ? "Stable" : "Legacy";
+	this.label = this.stable ? "Stable" : options[ "pre-release" ] ? "Pre-release" : "Legacy";
 	// 1: Always with a trailing slash
 	this.path = path.replace( /\/*$/, "/" ) /* 1 */;
 	this.pkg = JSON.parse( fs.readFileSync( path + "/package.json" ) );