Audio decoder element structure is based in `gst-plugin-lewton` (a lewton based Vorbis decoder created by @slomo)
The element assumes correctly parsed input from `flacparse`.
Implementation details:
* Claxon returned frames does not contain audio channels interleaved, the reorganization of the channels is done by the element.
* Claxon output buffers are always Vec<i32>, mapping to the correct type (depending on the audio format) is also done by the element.
Mono s16 and stereo_s32 test are provided.
Complete pipelines to test are:
```
gst-launch-1.0 -v souphttpsrc location=https://archive.org/download/MLKDream/MLKDream.flac ! queue2 ! flacparse ! flacdec ! audioconvert ! autoaudiosink
gst-launch-1.0 -v audiotestsrc ! audio/x-raw, format=S16LE, layout=interleaved, rate=44100, channels=1 ! audioconvert ! flacenc ! flacparse ! claxondec ! autoaudiosink
```
Fixes#71
