Add 'center', 'translate' and 'bg_color' arguments to 'rotate' FX

[mondeja]

• Added support for arguments center, translate and bg_color (in PIL is named as fillcolor).
• Add PIL version checking to make sure that arguments are supported by the PIL version of the user.
• Fix documentation.

• 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
• I have formatted my code using black -t py36

[coveralls]

Coverage increased (+0.04%) to 66.812% when pulling 41da302 on mondeja:better-rotate-fx into 8cc3eeb on Zulko:master.

[mondeja]

I'm not sure if bg_color is the name that would use moviepy. What do you think @tburrows13?

[tburrows13]

Just fyi for when you are updating docstrings bear in mind that we are using numpydoc (https://numpydoc.readthedocs.io/en/latest/format.html#sections), and in my doc improvements PR (#1438) I'm adding things like optional and argument type (as in numpydoc standards) to the parameter documentation. Feel free to do the same when you are adding new documented parameters.

bg_color is what is used elsewhere in moviepy so you should use that for now. I didn't change it in #1170, but maybe it should be changed in the future, but if that happens, we can easily change this one along with the rest.