Fix Poly(A)-trimming rule

In labkey_to_snakemake.py fixed the parameters so that there is 3p as well 5p polya
feature for every mate, which can be matched to the -a -g -A and -G options of cutadapt
depending on which is the sense or antisense mate the appropriate variable is populated
and the rest of variables are filled with 'XXXXXXXXXXXX' which leads to no trimming by
cutadapt. The poly-A trimming rules are fixed to contain all -a -g -A -G options.