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.
Use shell=False and a list rather than building a string and shell=True

There's no reason that I can see for using shell=True here, and it brings in
a number of risks (what if a file name has quote characters in it?). This is
safer and possibly faster, and arguably more readable as well.
Merge pull request #35 from Rosuav/popenshellfalse

Use shell=False with Popen, add monkey-patch-ability to FFMPEG calls.

(Thanks!)