Implement separate "5th string capo" for banjo

[kamalmostafa]

(Assuming the implemenation of the banjo fifth string per #26 ...)

A capo on a 5-string banjo does not affect the 5th string (at least, when placed at frets 1 through 5). That said, it is not unreasonable that Fretboarder "assumes" that when using a capo on a 5-string banjo that the 5th string will (somehow) be tuned up by the same number of frets. However, a nice feature would be separate "5th string capo" parameter to account for various arrangements of railroad spikes or other mechanism.