Automate .gitignore handling

[GantMan]

When rubberstamp is installed and the _base files are generated, the stamped versions should be added to the .gitignore file so it doesn't nag and cause binary merge conflicts.

resources/Icon-72.png
resources/Icon-72@2x.png
resources/Icon-Small-50@2x.png
resources/Icon-Small@2x.png
resources/Icon.png
resources/Icon@2x.png

Upon rubberstamp:revert the designated lines should be removed from .gitignore

NOTE - Verify this actually is true, lol

[mattsgarrison]

If someone already had icons in place and committed to their repository before adding motion-rubberstamp this would need to remove the original icons from the repository for the gitignore entries to be observed. While I don't think it's overly invasive to add and remove lines to a gitignore file, altering someone's existing repository could be an unwanted surprise.

Perhaps if no icons exist yet, we could add the gitignore entries. I'm not really keen on invoking git commands for the user, people are really picky about their git workflows.

Thoughts?