interactive_open() takes a target and an optional command, if it does not
receive a command then it uses open_anything().
It parses command and lexes it with shlex.split(), revieling the client
from that task.
"config -e" command uses it, and gives a better error message in case of
problem. "play" plugin uses it as well, as side-effect being that the
command is now interactive, as requested in issue #1321.
Fix issue #1321.
Include import of __future__ features division, absolute_imports and
print_function everywhere. Don't add unicode_literals yet for it is
harder to convert.
Goal is smoothing the transition to python 3.
