add command line option '-p'

Usage:

```
$ java -jar libbun-0.1.jar -p py


LibBun0.1 (Reference Implementation of D-Script) on Java JVM-1.7.0_45
Copyright (c) 2013-2014, LibBun project authors
Accept: zen-0.1 zen-trycatch-0.1 python 
Produce: Bun-0.1
>>> 

```