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