shared 'special keyword arguments' feature with test

:) may be you will find it usefull. For 'special keywords arguments' you have chose good defaults, but some times it is needed in some context some other defaults... With this feature it is possible in next way:
```python
import sh
from StringIO import StringIO
buff = StringIO()

shb = sh(_out=buff)
shb.ls('-l')
#the same as sh.ls('-l', _out=buff)
```