This allows users to use different configurations than the one provided
by Jami. File settings overwrite Jami's existing configuration options.

Note that only codec-level options are supported, format-level is not
implemented.

The following options are ignored, as they are set by going into the
settings page: width, height, framerate, sample_rate, channels,
frame_size, parameters (internal option for h264's profile level id).

If a file encoder.json exists in the same folder as dring.yml, it is
loaded into the encoder's configuration options. The config file is
separated by codec. Multiple configurations can exist for the same codec
family (ex: libx264 and h264_vaapi) because each encoder has different
options. Section name should be the codec implementation's name and not
the codec's name (h264 won't work, but libx264 will).

Example configuration file:
{
    "libx264": {
        "crf": 15,
        "preset": "ultrafast"
    },
    "h264_vaapi": {
        "low_power": 1
    },
    "libopus": {
        "application": "voip"
    }
}

Change-Id: Ia6bb184af382638d4016256b393ac7295408ed82
Gitlab: #75
Reviewed-by: Sébastien Blin <sebastien.blin@savoirfairelinux.com>