title: Specify lag in numpy.correlate
body: <p>Matlab's cross-correlation function <code>xcorr(x,y,maxlags)</code> has an option <code>maxlag</code>, which returns the cross-correlation sequence over the lag range <code>[-maxlags:maxlags]</code>. Numpy's  <code>numpy.correlate(N,M,mode)</code> has three modes, but none of them allow me to set a specific lag, which is different from full <code>(N+M-1)</code>, same <code>(max(M, N))</code> or valid <code>(max(M, N) - min(M, N) + 1 )</code>. For <code>len(N) = 60000</code>, <code>len (M) = 200</code>, I want to set the lag as 100.</p>