OptionLoader: add flags to loadOptions

* 2 new params:
	* req_flags
	* rej_flags
* Can manage specific AVOptions depending on their flags.
Option: delete Option subclasses

* Option class is not abstract anymore: it represents all possible type
of Option (Int, Boolean, Choice...).
* OptionType enum manages the different cases of option (Int, Boolean,
Choice...).
* Option class contains an array of Option: contains sub options in case
of Choice or Group.
* OptionLoader manages an array of Option (instead of pointers).
* OptionChecker doesn't need to include libav / ffmpeg files
(thankfully).
* Update SConstruct: Options folder doesn't exist anymore.