A `stdin` of `None` means the ffmpeg subprocess input inherits from the
parent process, which may cause undesired control from a terminal (e.g.
`C` causes ffmpeg to prompt for a command). It also closes the parent's
stdin when the subprocess exits.
This commit switches to `subprocess.DEVNULL`, which provides a separate
pre-closed stdin for ffmpeg subprocesses.
`Webhook.send()` now accepts a `files` kwarg holding a list of `File`
objects, which are included in the HTTP request as `file1`, `file2` and
so on.
This is an undocumented feature of the Discord API, but is analogous
with the client's sending of messages with multiple files.
TerranceandRapptz