disableColoring and enableColoring methods added to IceCreamDebugger closes#155

[fanoway]

ic.disableColoring()
ic.enableColoring()

Based on this context manger taken from the unittests:

@contextmanager
def disableColoring():
    originalOutputFunction = ic.outputFunction

    ic.configureOutput(outputFunction=stderrPrint)
    yield
    ic.configureOutput(outputFunction=originalOutputFunction)

Note: I had to modify tox.ini and ci.yml to make the tests pass due to deprecated support for python 2.7