Add support of non-0 Nyquist to fftfreq?

[N5N3]

Now fftfreq only return the frequency axis on the 0th Nyquist Band by default.
Maybe we could add a argument, i.e. fc, to determine where the axis is.

[galenlynch]

That's a good idea, but this function was moved to AbstractFFTs.jl. As such, could you please repost this issue in AbstractFFTs.jl? I will close it here in DSP.jl.