Accept format argument to build command

lib/index.js

@@ -9,12 +9,16 @@ module.exports = {
 	Book: Book,
 
 	commands: [
+		// Build command that simply build a book into an output folder
 		{
 			name: "build",
 			description: "build a book",
 			exec: function(args, kwargs) {
 				var input = args[0] || process.cwd();
 				var output = args[1] || path.join(input, "_book");
+				kwargs = _.defaults(kwargs || {}, {
+					format: "website"
+				});
 
 				var book = new Book(input, _.extend({}, {
 					'output': output
@@ -22,10 +26,12 @@ module.exports = {
 
 				return book.parse()
 				.then(function() {
-					return book.generate();
+					return book.generate(kwargs.format);
 				});
 			}
 		},
+
+		// Install command that install plugins needed by a book
 		{
 			name: "install",
 			description: "install plugins dependencies",