add support for cut-length parameters to trim-* actions

[gregcaporaso]

Forum x-ref: https://forum.qiime2.org/t/is-it-possible-to-truncate-trim-right-3-end-without-filtering-by-length/30983/9

[lizgehret]

similar functionality has already been implemented in cutadapt: #57

[gregcaporaso]

Thanks @lizgehret, I forgot about that one. It looks like that is only on the demux- actions. For this user's use case it would need to be added to the trim- actions as well.