Add Afterburner option for FDK AAC

[kylagit]

Is your feature request related to a problem? Please describe.
There is a setting in FDK AAC called Afterburner, which when enabled produces better quality audio files, however there is no way to turn this on in fre:ac.

Describe the solution you'd like
An option to turn on Afterburner when using the FDK AAC encoder.

Describe alternatives you've considered
Using other software, but none can compete with fre:ac.

Additional context
Fraunhofer themselves recommend turning on Afterburner.

[enzo1982]

The FDK-AAC Afterburner feature is already enabled in fre:ac. See here.

There is currently no option to disable it, though, but it might make sense to add one. Without Afterburner, FDK-AAC is approximately 2.5x faster.

[kylagit]

@enzo1982 Thank you! However it would be great to add the option to switch between CBR/ABR/CVBR/TVBR modes for the Apple encoder.