Suggestions for more expressive naming for config variables "file" and "file_format"

The current settings to choose the audio source are called file and file_format. This is used for both, audio devices and audio streams as the source for recordings.

This naming scheme might be a bit misleading, since it's actually not so much about files:

# More information about the options can be found in OPTIONS.md
Audio:
  input:
    # force input file format, for example "alsa"
    file_format: "alsa"

...
    # input file to listen to, can be a pcm device or a stream
    # for example "dsnoop:CARD=USB,DEV=0", "http://stream.freirad.at:8002/live.ogg", "AuRa_recording"
    # leave the input file blank for a cli selection of a pcm device
    file: "default"

Suggestion:

• Use source to either identify a stream like source: "http://stream.freirad.at:8002/live.ogg" or a device source: "dsnoop:CARD=USB,DEV=0". Set it as the first item of the config, since it's most likely to change.
• Use source_format with alsa or the stream format mp3, ogg