Add an .editorconfig.

An `.editorconfig` file will make it easier for people to contribute according to the guidelines.

I put in a pretty small example that covers what @andsens outlines in the `CONTRIBUTING.md` file as well as a couple additional "sensible" suggestions to show the sorts of things you can do. I would be happy to adjust the settings to whatever is preferred.

I also updated the `CONTIBUTING.md` file to reflect the addition of the `.editorconfig` and formatted it to comply with the guidelines set in the `.editorconfig`.