Tweaks and fixes

[almarklein]

• Support for pathlib.Path (and black formatting)
• Sets default ffmpeg_timeout to 0 (wait without timeout). Closes #30
• Prevent sigint from propagating to ffmpeg. Closes #4
• When interruping (ctrl-c, KeyBoardInterrupt, SystemExit) during reading or writing, we terminate ffmpeg without waiting.
• Make that we actually close ffmpeg nicely after reading, without hanging. Related to #17 and #19.
• This also closes #20 (technically #19 already fixed the segfault, this PR makes sure we terminate ffmpeg in a nicer way).

This also:

• Fixes https://github.com/imageio/imageio/issues/482
• Probably fixes https://github.com/imageio/imageio/issues/489

[almarklein]

cc @zulko: this PR fixes a few long-standing issues that your users may also have had problems with.