Add change_duration flag to Clip.set_fps()

[mxbi]

This pull fixes #1069 by adding a new change_duration=False flag to Clip.set_fps. When False, original behaviour is conserved, When true, the speed and duration of the video are both changed when the fps is changed (so that all frames are preserved 1:1).

I have yet to write tests but on some test videos the behaviour is as expected.

• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it
• I have formatted my code using black -t py36

[coveralls]

Coverage increased (+0.05%) to 67.532% when pulling d746418 on mxbi:fps-changeduration into 3a6f014 on Zulko:master.