Add `/.editorconfig`

## See: http://editorconfig.org/
This is not an issue with editor. It really is an issue with that usually git will checkout with the system core EOL. This can be changed inside `.gitconfig` with the `core.eol` config, but I don't think there's a way to **share** that config. (maybe when [set explicitly via `.gitattributes`](http://stackoverflow.com/questions/2332349/best-practices-for-cross-platform-git-config/2361309#2361309))

The issue is if your code is checked out with CRLF, then the editor will convert to LF on opening, meaning `git diff` become unusable on windows (because every line changed).

So, if H5BP wants to enforce LF in `.editorconfig`, then it **must** change it's `.gitattributes` file because right now it enforce checking out CRLF on windows.