Number exercises on two digits to preserve order in file system

The goal is to avoid exercise10- coming before exercise1-, exercise2-...
which is shown in this order in Ubuntu/Linux shell.
Add code and text file to generate Bach Crab Canon (start of exercise 11)
Add parameter to crab_canon for input/output file name

This will allow to generate multiple crab canons using the same function
over the course of exercise 11.
Add generated Crab Canon from Bach's Musical Offering