Adds -f flag to use a custom CSV [f]ile of barcodes.

[sclamons]

Adds the ability to use custom barcodes defined in a (user-specified) CSV file using a -f flag.

If the flag is on, the first thing porechop.py does is to parse the CSV file into a list of Adapter objects, then append that list to the end of the list of Adapters from adapter.py.

[kyrubas]

@rrwick, first off- great repository. Second, any thoughts on merging this into the main repo? As I would rather not alter the hard-coded adapters file, I'm going to clone this fork for now as it solves my use case.
@sclamons, what are your thoughts on adding a second argument to completely over-ride the default adapters rather than append them to the list? Or even having the defaults as a default csv that gets generated into a list of adapter objects rather than a *.py file.