Added the ability to read the mean and max volume level output

[mcyoung]

Added in a way of grabbing the mean and max volume level output from the FFMPEG command line. This allows audio to be adjusted with the tool, for example, matching audio levels for 2 video files.

[bikeath1337]

This metadata should be present in the Movie's metatdata attribute as of 2.1.0.

[marina101]

Hi,
I checked both version 2.1.0 and the most recent one, but can't find the mean volume or max volume attributes in either the metadata or anywhere else on my movie object. Didn't see a specific reference to them in the source either. Can you please point to where they are? Is it possible they're not included in the general metadata dump of the movie, but need to be accessed with the specific ffmpeg script (like the one in this PR)? The only way I was able to access it for my movie file was by running that same ffmpeg script (ffmpeg -i audiofilename.mp3 -af "volumedetect" -f null /dev/null), but it would be great to be able to access it via the gem directly.