Open a file
filename as a IOChannel using mode
This channel will be closed when the last reference to it is dropped, so there is no need to call
doing so will not cause problems, as long as no attempt is made to access the channel after it is closed).
A string containing the name of a file
One of "r", "w", "a", "r+", "w+", "a+". These have the same meaning as in open
A IOChannel on success, null on failure.