Adjust sorting by mutations to new snp format

[wm75]

This fixes the error with v1.1.2 and --sort-by-mutations:

line 355, in make_graph
    if int(record_mutation[:-2]) == int(sort_mutation):
ValueError: invalid literal for int() with base 10: '1059:'

[wm75]

Not sure all the different ways of sorting would play well together with indels.

[corneliusroemer]

Can you make a new release please? I also encountered this error just now :) Snipit is rad!