feature request: add slice

[sa-lee]

so we can find max/min correctly as per https://stackoverflow.com/questions/24237399/how-to-select-the-rows-with-maximum-values-in-each-group-with-dplyr

as requested in https://github.com/mikelove/plyrangesTximetaCaseStudy/issues/2

[sa-lee]

closed from #72