Update gitignore with thumbs and mac specific files

[dingo-d]

The .DS_Store started popping up when I wanted to check out certain branches, so this will just ignore it if a person is working on a Mac computer. If you have any additional Windows-specific file types to exclude let me know in the comment.

[grappler]

This standard files should be added to your global .gitignore file on your local machine. https://help.github.com/articles/ignoring-files/#create-a-global-gitignore

If this really should be added then this is should be added upstream as any changes we make to the files that are in WPCS may cause a conflict when they get changed in the future.

[dingo-d]

Some people might not have global .gitignore set up (I didn't :smile:), like first time contributors. If this can be fixed upstream, then I'm all for it :+1:

[jrfnl]

I agree with @grappler in that this should be set in a users global .gitignore.

This is not the kind of repo where we expect total code newbies to contribute with code, nor is WPCS itself, so I would suggest closing this PR (and not pulling it to WPCS).

[dingo-d]

Sure thing :+1: