Cmd2func

* Support pass kwargs to Popen

**Full Changelog**: https://github.com/Nanguage/cmd2func/compare/0.1.5...0.1.6

+ Support decorate generator function

Support use as decorator.

+ Improved the reusability of the cmd2func class.
+ Changed the way stdout / stderr are captured.
+ Added the Tee class, which allows for capturing stdout / stderr and printing to the terminal at the same time.