Handle encoding/decoding
    
    When writing files or issuing commands, bytestrings are passed through
    unchanged, and text strings (`unicode` in Python 2) are encoded using
    a configurable encoding (`utf-8` by default).
    
    When reading files, bytestrings are returned by default,
    but an encoding can be given to get a test string.
    
    For command output, separate `stdout_bytes` and `stdout_text` attributes
    are provided.
    The latter uses a configurable encoding (`utf-8` by default).
    
    Fixes: https://pagure.io/python-pytest-multihost/issue/7