Add pattern for optional execution using `until` operator

[evanfloden]

See example from @micans

ch_fastqs_cram
.mix(ch_fastqs_dir)
.into{ ch_rnaseq; ch_fastqc; ch_mixcr }

ch_rnaseq
.until{ skip_align_step }
.into { ch_star; ch_hisat2; ch_salmon }