Add support for pathlib

It would be nice to add support pathlib, as others (such as NumPy or PIL) do. Converting all input path arguments to str would probably be enough.

[anisayari]

Update : I was able to use pathlib with just the as_posix() method :

avi_files_list = pathlib.Path('path_to_my_avi_files_folder').glob('*.abi')
for anv_file in avi_files_list:
    clip = VideoFileClip(anv_file.as_posix())

[tburrows13]

Done in #1137. At the moment, moviepy just converts the path object to a string as soon as it receives it. In the future it would be nice if it internally used Path objects until as late as possible down the pipeline, but that would require a larger rewrite which seems unnecessary seeing as it works fine currently.