Add EditorConfig files to define indentation styles

These `.editorconfig` files define the indentation style used in the Ruby files and in files used in the classic theme.

[EditorConfig][] is a file format for defining coding conventions in shared projects.  EditorConfig plugins are available for [many popular text editors][plugins].

With these `.editorconfig` files present, code contributors with an EditorConfig plugin installed will find adhering to your indentation style more natural.

--

*Note:* I co-created the EditorConfig project last year.  I greatly appreciate feedback on the project and I would like to hear any ideas or concerns regarding EditorConfig.

[EditorConfig]: http://editorconfig.org "EditorConfig homepage"
[plugins]: http://editorconfig.org/#download