After creating the virtual environment, we should add `<VENV_DIR>/bin`
to the `PATH` variable, so any next `python`/`pip` commands will be
executed in the new environment.
Before this fix, `platyvenv`, unlike `platydock`, didn't take into
account any extra before/after installation commands that individual
integrations may instruct to run in their manifest files.
The two scripts now share the same command interface, behaviour and base
class.
Also, Platydock now builds a Docker image instead of just printing a
Dockerfile, unless the `--print` option is passed.
Fabio Manganiello