pgmap CLI

[marissafujimoto]

Issue Description

Right now #11 implements pgmap as a python library. We want to have a CLI which provides a more convenient wrapper for pgmap.

Right now I'm imagining this as a one line command taking in fastq files, barcodes, and some kind of representation of the paired guide library and writing to a counts file. It could be distributed through conda or some other open source channel. I'm not too familiar with how python CLIs are made or distributed, though I know argparse is one typical library for implementing them.

I think we can take some inspiration from mageck for how bioinformatics software users would expect pgmap to behave: https://sourceforge.net/p/mageck/wiki/Home/