Add more options to the configuration file

[Lyamc]

Problem
Currently, the ffmpeg path is hardcoded (as of 0.3.0-rc6) and can not be changed after compile. This will create a lot of issues once distro packages are created.

Solution
I suggest a change so that the ffmpeg/ffprobe available in the $PATH, unless overridden with the config file, unless overridden with an environment variable.

Current workaround
A simple $ sudo ln -s $(dirname $(which ffmpeg)) /usr/bin/utils resolves the issue.

[Lyamc]

This pull request seems to cover this: #589 so I'll close this in advance