Use STDIN in nbstripout if no input is given

nbstripout.py can then be used as a clean/smudge filter in git:
http://git-scm.com/book/ch7-2.html#Keyword-Expansion

See this bugreport for an use case: https://github.com/ipython/ipython/pull/3291

Howto:
Add this to your `.git/config`:
```
[filter "stripoutput"]
	clean = "python c:/path/to/nbconvert/nbstripout.py"
```
and a `.gitattributes` file with
```
*.ipynb filter=stripoutput
```