[feature request] umi-edit-dist parameter

Thanks for the great software! I was trying to run this with a custom chemistry and umi-edit-dist parameter of 0, but was failing as simpleaf doesn't have the umi-edit-dist parameter of alevin-fry exposed, and alevin-fry doesn't appear to allow for custom chemistries. 

Would it be possible to expose the umi-edit-dist parameter in simpleaf? Thanks in advance!