Add HRTF based crossfeed.

Uses FFT convolution to produce crossfeed given two (left and right) stereo
impulse responses. The impulse responses should represent the sound heard by
the ears when the sound is emitted by left and right, respectively, virtual
loudspeakers.
Add fftw3 to dependencies in README.md.
Cleanup crossfeed_hrtf initialization.
Fix mistake in last patch.
Make format detection based on file extension more complete.