add flush=True for progressive stdout in recursive processes and cont…

[circle-queue]

…ext manager

add venv flag to simplify run()
error if path (venv) doesn't exist

TESTS:
Quick example:

python -c "from makefile import run;     run(\"\"\"python -c \"from makefile import run; run('echo 1 && timeout 2 && echo 2')\" \"\"\")"

Practical example:

python -c "from makefile import run;     run('python makefile.py venv && python makefile.py build && python makefile.py clean-all')"