Config value for barcode mismatch threshold in demultiplexing rules

[cnluzon]

cutadapt paramer -e right now is hardcoded to 1 mismatch. I think depending on the barcode pool minimum distance we might want to be able to control this to rescue a few more reads per barcode. The priority for this is very low for us, most of the time we are OK with this setting. But this one feels like a parameter to config, also because I think it's best not to make assumptions about the barcode pools other potential users could have.