Version 0.3.1

Improvements

• Adds build_slug/2 to accept the original changeset as the second argument, it still receives list of sources as the first argument
• Updates build_slug/1 inner logic

Documentation

• Updates README.md with the new example
• Updates docs to handle new changes
• Updates CONTRIBUTING.md with 'Development' section

Testing

• Adds new test cases to cover build_slug/2