Redesign codec

Resolves https://github.com/pion/mediadevices/issues/114

* Remove codec registrar
* Completely redesign how codec is being discovered, tuned, and built
* Update examples
* Update unit tests

pkg/codec/codec.go

@@ -8,5 +8,35 @@ import (
 	"github.com/pion/mediadevices/pkg/prop"
 )
 
-type VideoEncoderBuilder func(r video.Reader, p prop.Media) (io.ReadCloser, error)
-type AudioEncoderBuilder func(r audio.Reader, p prop.Media) (io.ReadCloser, error)
+// AudioEncoderBuilder is the interface that wraps basic operations that are
+// necessary to build the audio encoder.
+//
+// This interface is for codec implementors to provide codec specific params,
+// but still giving generality for the users.
+type AudioEncoderBuilder interface {
+	// Name represents the codec name
+	Name() string
+	// BuildAudioEncoder builds audio encoder by given media params and audio input
+	BuildAudioEncoder(r audio.Reader, p prop.Media) (io.ReadCloser, error)
+}
+
+// VideoEncoderBuilder is the interface that wraps basic operations that are
+// necessary to build the video encoder.
+//
+// This interface is for codec implementors to provide codec specific params,
+// but still giving generality for the users.
+type VideoEncoderBuilder interface {
+	// Name represents the codec name
+	Name() string
+	// BuildVideoEncoder builds video encoder by given media params and video input
+	BuildVideoEncoder(r video.Reader, p prop.Media) (io.ReadCloser, error)
+}
+
+// BaseParams represents an codec's encoding properties
+type BaseParams struct {
+	// Target bitrate in bps.
+	BitRate int
+
+	// Expected interval of the keyframes in frames.
+	KeyFrameInterval int
+}