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, `/.editorconfig` includes some basic properties that
reflect the coding styles from the files provided by default, but
users can easily change them to better suit their needs.

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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