Get decoder by name

[gBillal]

Hi @mickel8, just skim through the PR and see if you're ok with everything.

Summary of the PR:

• Created a new Xav module and nested all the other files inside a xav folder (typical elixir project structure)
• The Xav module contains functions to list pixel formats, sample formats and decoders to mirrors the ffmpeg commands.
• Use NimbleOptions to validate decoder options
• Made the decoder fetch codecs by name, this will make the decoder capable of decoding any video/audio that ffmpeg can decode instead of adding support for each codec separately.

Some points I avoided in this PR but worth discussing:

• Make the decoders returns a list instead of :ok/{:ok, frame} (like the encoder). What do you think ?

If you have no problem with all of this, can we release a new version ?

[mickel8]

Re decoders, let's leave this for another PR. If decoder code can indeed return multiple frames then we probably need to do this

[mickel8]

Awesome! Thanks a lot 🎉 no blocking comments from me

[mickel8]

Once merged, I will make release :)