Feature request: trim based on user defined length

[bwprice]

It would be really helpful if there was an option to trim the first X and last Y bases as this would really help remove the junk that Sanger seq end up having here, leading to fewer ambiguous bases in the resulting consensus. For example in COI barcoding I find trimming each sequence to 650bp and then trimming the first 100bp before assembly removes 99% of the issues. Assembling the two 550bp sequences still results in a 650bp contig, but with far fewer Ns and clean sequence across the contig, admittedly with 100bp overhangs.