Replace 'auto' with 'unset' per the new spec

[alaycock]

To make the maintainer's life easier, I...

• [x] created at least 1 spec to cover my changes,
• [x] npm tested my code and it's green like an :green_apple:,
• [x] adjusted the readme.md, if it was necessary and
• [x] would like to receive get a :beer: or :coffee: after that hard work!

Resolves https://github.com/sindresorhus/atom-editorconfig/issues/193

I updated the grammar and changed all relevant instances of auto to unset per the new terminology. If you search the repo, there are a few instances of auto left, but those referring to Atom's built in 'tab type' setting. It made more sense to update every instance of auto to unset to prevent conflicting terminology throughout the repo, although it isn't necessary to do so.

I also added a new test, to show that when any of the properties are unset, the settings in the plugin will match that.

[florianb]

Well done @alaycock!

Your changes look good to me, i will have a deeper look at Tuesday and i guess i will then accept the PR, too. Thank you very much for your contribution!

🎁

[florianb]

I owe you a beer @alaycock - thank you very much for your support! 🍻

[alaycock]

Glad I could help 🍻