Merge: niti: teach the interpreter to read from the standard input

With `-` the program is read from stdin.

~~~
$ echo H4sIAN7lUFcAAysoyswrUVDySM3JyVfiAgCjfiaWDgAAAA== | base64 -d | gunzip | nit -
Hello
~~~

Pull-Request: #2159
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>