Add 'start_time' and 'end_time' parameters to multiply_volume FX

[mondeja]

This introduces the optional arguments start_time and end_time for multiply_volume FX. The volume multiplication, if these parameters are not None, is only applied to the seconds in the range from start_time to end_time. Note that after this, is a lot easier to introduce silences in a clip: clip.multiply_volume(0, start_time=5, end_time=10). The code is totally backwards compatible. I've also properly documented the function and rewritten the tests parametrizing them.

Closes #1571

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
• 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

[coveralls]

Coverage increased (+0.2%) to 69.492% when pulling ecd43d6 on mondeja:multiply_volume-start-end into 978c328 on Zulko:master.