Enable reading metadata

[Borloxos]

I currently have the need to read the metadata of an input file, as supplied by ffprobe, e.g. to transcode to different output sizes, depending on the input size of a file.

As this is currently not possible with this library, I tried to implement the changes necessary for that.
The changes include

• Creating interfaces for Metadata, Format, Streams, Tags and Disposition, that contain all the necessary getters
• Moving the Metadata type definitions in package ffmpeg into their own file, together with implementations of the necessary getters
• Exporting the function Transcoder.getMetadata() and making it available inside the Transcoder interface
  • Along with this, changing some types and calls in ffmpeg/ffmpeg.go

I've tested reading the metadata with the following code snippet, that demonstrates the changes capabilities:

transcoder := ffmpeg.
	New(ffmpegConfig).
	Input(transcoderRequest.Filename)

metadata, err := transcoder.GetMetadata()
errorHandler.HandleError(err, "component", "transcode", "function", "transcode", "action", "transcoder.GetMetadata")
fmt.Printf("Duration: %s\nFilename: %s\nFormat: %s\nProbeScore: %d\n\n", metadata.GetFormat().GetDuration(), metadata.GetFormat().GetFilename(), metadata.GetFormat().GetFormatLongName(), metadata.GetFormat().GetProbeScore())

for _, element := range metadata.GetStreams() {
	fmt.Printf("Codec-Name: %s\nHeight: %d\n\n", element.GetCodecName(), element.GetHeight())
}

This PR should not break any existing behaviour. If there are any problems with it, I'll be happy to fix them.

[denisonfire]

Very useful feature. Looking forward it will be merged soon.

[xfrr]

Very helpful, thanks!