Subby

* Expose `stdin_stream` property on `Process`

* Check for redundant/invalid popen kwargs
* Enable default values for `timeout` and `raise_on_error` to be passed to the `Processes` constructor

* Bugfixes

* Change `sub` to take either a command or sequence of commands

* Added mode parameter to specify raw (bytes) vs text mode
* Added `sub` convenience function
* Added tests
* Blackened code

* Add ability to specify alternative allowed returncodes