Add .editorconfig

I added a default .editorconfig for my fork. Since the project is big on adhering to standards, an `.editorconfig` makes doing that really easy and would be good to include. If the dev's IDE isn't configured for it, nothing changes, so it's not a requirement or dependency, but just works for those it works with.

I also have a fork that scaffolds unit tests as well. It's basically just the result of running `wp scaffold plugin-tests` but I can open a PR if you'd like that as well. There isn't an functionality in the Boilerplate that I would test, now that the multisite logic from the previous version is gone. Do you want that too or no?