Optional disabling of `prevent_sigint`.

[Bulkin]

This is a workaround for a strange hang on fork() when Popen is called with a preexec_fn (see #58). If the environment variable IMAGEIO_FFMPEG_NO_PREVENT_SIGINT is not empty, preexec_fn is set to None.

[almarklein]

Thanks! Three comments.

• It would be nice if "0" and "false" also resolve to Falsy. See e.g. this discussion. A quick version could be something like os.getenv(...).lower() not in ("", "0", "false").
• Would you mind adding a section before the API section in the README that lists the environment variables? This one and IMAGEIO_FFMPEG_EXE. Otherwise I can do that after merging this PR.
• Could you run black . on the code to fix formatting? :)

[Bulkin]

Done.

[almarklein]

Thanks!