Exporting get_pileup()

[mikemc]

Currently, plot_pileup() is exported, and depends on get_pileup() to generate it's first argument; yet get_pileup() is not exported. If the intention is for plot_pileup() to be user-facing, then it seems that either get_pileup() should also be exported, or plot_pileup() should take a file as its argument instead of a pileup. My vote would be for the first option, to export get_pileup(), as I personally find it useful.