Add option to pass utf8 parameter to latexdiff.

Before adding this option, latexdiff wasn't using utf8 (though it's supposed to be the default) on my .tex docs, thereby breaking things like em–dashes. This PR adds a --utf8 option and is designed to allow similar future additions to require only two changes (i.e., an added statement to `parse_arguments` and an added test and string concatenation in `main()`.

Also, Atom (text editor) ate some whitespace on empty lines.