Allow the name en-ffmpeg to be changed via monkey-patch

This allows non-root users to choose which ffmpeg to execute, without any need
to manipulate $PATH, and may also allow the use of avconv.

Full support for avconv probably just requires that someone try every parameter
you're giving ffmpeg to see if avconv supports it, and if it does the right thing.

It'd be nice to support avconv - some systems (eg Debian GNU/Linux, which is
what I use) don't ship ffmpeg.