Automatically ignore anything in .gitignore

[miketheman]

Checklist

• This is a new feature, not a refactoring suggestion
• I have checked for this feature in the Sourcery documentation
• I have checked there are no open requests referencing the same feature

Description

Somewhat following #63

Instead of duplicating the entries in .gitignore, add them to the ignore section dynamically.

Maybe take advantage or inspiration from https://pypi.org/project/gitignore-parser/