Handle polyA processing in input preparation script

• fixes some functions in labkey_to_snakemake.py
• add optional argument for trimming polyA tails; they are trimmed as follows:
  • if mate is sense, oligo-A is added to sample table for cutadapt rule to trim
  • if mate is antisense, oligo-T is added to sample table for cutadapt rule to trim
  • if option is set to --trim_polya, oligo-X stretch is added to sample table and cutadapt will not trim

Closes #37 (closed)