OptionLoader: add format and codec context attributes

* In loadOptions function, choose what ffmpeg / libav object on which
we'll scan AVOption: avFormat or avCodec.
* Update OptionChecker app: no need to get a codecContext from the
audioDesc of a stream of an InputFile !
Description: update getInputExtensions

* Also parse name, because name's format defines (in general)
extensions.
Description: update getInputExtensions

* Sort array of extensions.
* Suppress duplicates.