Improve the `dotfiles` script

* Handle command-line arguments better.
* Improve code quality by removing repetition where possible.
Make `dotfiles` ask for `sudo` password only once

Ref: https://gist.github.com/cowboy/3118588
Update `.editorconfig`

Add property in the `.editorconfig` [1] file that prevents the trimming
of trailing whitespaces for Markdown (`.md`) files.

This is done because Markdown uses [2] the trailing whitespaces: "When
you do want to insert a <br/> break tag using Markdown, you end a line
with two or more spaces, then type return."

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[1] http://editorconfig.org/#supported-properties
[2] http://daringfireball.net/projects/markdown/syntax#block