Add new option `--cutadapt_args` to pass on additional params

[FelixKrueger]

It seems like a good idea to pass on additional parameters to Cutadapt, e.g. --poly-a. In the current form, this can only be done via the `-a "SEQUENCE --additional_param" workaround, however this has the drawback of by-passing the adapter auto-detection. This sounds like a job for half-term....