Implement Default for Settings struct #72

Can I implement Default for Settings struct? I want to build more tests for this app, on this app is important to know the input/output of certain key functions. For example this one: print_unicode, you send it a GitGraph as input argument and it outputs a triplet of String<Vec>

By having the tests we can have living documentation on what every function has. A Default implementation for Settings will help me to build test data on the tests.