Suggestion: Allow selection of ffmpeg binary

[Azrael-1123]

There are some valid reasons to have multiple ffmpeg installs at once (for example, working on ffmpeg itself and/or keeping older versions for compatibility testing) - I thought about adding a command-line flag that would set which ffmpeg to use (defaulting to just ffmpeg and relying on path when not set), however I'm worried it might confuse non-technical users even more than we already do to see that in the usage help

tl;dr i'm asking for some opinions on whether this might be worth implementing