Add `/.editorconfig` file

The addition of the `/.editorconfig` file is made in order to
encourage and help developers and their teams define and maintain
consistent coding styles between their different editors and IDEs.

By default, the `/.editorconfig` file includes some default
properties that apply to the coding styles used by HTML5 Boilerplate,
but users can easily change them to better suit their needs.

Since users need to install a editor/IDE plugin in order for the
properties from the `/.editorconfig` to be applied, we were reluctant
to add this file in the past (see: h5bp/html5-boilerplate#1124).
Nowadays however, the EditorConfig project provides plugins for most
of the popular editors/IDEs (see: http://editorconfig.org/#download),
plus, more and more developers are using it.

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Ref: http://editorconfig.org/

Close: h5bp/html5-boilerplate#1561
       h5bp/html5-boilerplate#1564