Added ffmpeg download when importing moviepy.editor

[tburrows13]

See discussion in #493. Calling imagio.plugins.ffmpeg.downlad() doesn't install ffmpeg if it is already installed somewhere on the system.

[coveralls]

Coverage increased (+0.03%) to 61.807% when pulling 718879b on tburrows13:ffmpegdownload into 6b0710e on Zulko:master.

[coveralls]

Coverage increased (+0.2%) to 61.997% when pulling 27f6485 on tburrows13:ffmpegdownload into 6b0710e on Zulko:master.

[Zulko]

Definitely necessary, but would this cause a problem for people running moviepy without direct internet access ? Such users would certainly have their own FFMPEG_BINARY, so maybe add something like this:

if os.getenv('FFMPEG_BINARY', 'ffmpeg-imageio') == 'ffmpeg-imageio':
    imageio.plugins.ffmpeg.download()

[mgaitan]

could we merge this?

[tburrows13]

Yep thanks for the reminder.