"N" base handle

[shizhuom]

Hi,

Thanks for developing this helpful tool!

When I used l-giremi 0.2.3, I found that if there are "N" base in reads, there will be a problem in mismatch.py file that "N" is not in the dictionary. However, such problem did not occur in your previous version. I wonder could you integrate this situation in your script so that "N" base will no longer be a problem?

Thank you!