Generate .gitignore

[DannyPeck]

Hatch generates certain files as a part of its asset generation step, etc. These files should be added to a .gitignore file when a project is created.

[DannyPeck]

So currently we use Tup to generate a .gitignore file for all of the artifacts it creates; however, this is an issue because Hatch also generates the Tup files and we would like to add these to the .gitignore as well. (Since they can be regenerated from the Hatch.yml file, I believe they shouldn't be checked in). I can think of a few possibilities, 1) we stop using the Tup feature for generating .gitignore files and we generate them ourselves, 2) we find a way to append our entries to a .gitignore file if it already exists; however, I believe that Tup does not play well with this option, 3) we don't add the Tup files to the .gitignore all together.