Support for user provided presets and option overriding

Update: So I was having trouble generating proxies for 2.7k 4:3 GoPro footage and had to muck around with the presets to get the right one. Finally used mjpeg so all my earlier rant about hevc/h265 stuff is not relevant.

What is still relevant is if the following can be supported:

• enable reading of additional presets from an external user created file / command line
• allow overriding of common options if the same option is found in the user provided presets

Cheers