Update the encoder to allow accepting of media files of different codecs/types

[makstermaki]

The application should be updated to allow having media files of different codecs/types in the same channel.

Currently, a channel can only be made up of source media files that are the same type due to the ffmpeg command being used to start the stream. There are multiple ways this can attempted to be solved. One is the addition of functionality which preprocesses files into a standard media format can be added to allow the acceptance of any media type. Another is by updating the ffmpeg command used so instead of only copying the video and audio streams, it transcodes them. Note that a live transcoding solution would lead to a very large server processing load, especially if many channels would be running at once with transcoding on.