Don't hardcode output to output.mp4

[mre]

In #1 (comment), I mention that the output of this tool is currently hardcoded to output.mp4. See

timelapse/encoder.py

Line 21 in d9c15a0

self.output = "{}/output.mp4".format(output_dir)

.
That's bad, because a new recording will overwrite the previous one. We could fix that by adding a timestamp for the output file, for example.

Might be a nice exercise for new contributors.

[mre]

Fixed with #3