Optional disabling of prevent_sigint.

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.

imageio_ffmpeg/_utils.py

@@ -67,6 +67,11 @@ def _popen_kwargs(prevent_sigint=False):
             creationflags = 0x00000200
         else:
             preexec_fn = os.setpgrp  # the _pre_exec does not seem to work
+
+    if os.getenv('IMAGEIO_FFMPEG_NO_PREVENT_SIGINT', ''):
+        # Unset preexec_fn to work around a strange hang on fork() (see #58)
+        preexec_fn = None
+
     return {
         "startupinfo": startupinfo,
         "creationflags": creationflags,