Available file types for A/V should be dynamic based on what provider Aegisub is built with or is currently using

[OrangeChannel]

(An extension of #46)

I created #43 because ffmpeg supports opening the files, but Aegisub does not have them in the list of accepted extensions on the open video or open audio pop-up.

[2:46 AM] Ryan: it's probably worth going and making the extensions registered based on the providers available to people [2:46 AM] Ryan: the configuration flags should be imported into the installer

Basically,

• Aegisub/src/command/audio.cpp
• Aegisub/src/command/video.cpp
• Aegisub/src/project.cpp

Should be dynamic based on the provider Aegisub is built with, or even possibly the active provider chosen by the user in the Preferences > Advanced window.