Redirect output to selected file instead of standard output

[ftilmann]

This issue is taking up a suggestion in response to #195 made by @jonschz :

One could implement direct output to files like latexdiff --outfile=out.tex a.tex b.tex. I suspect this will be quite a bit of work to implement, though.

This could allow use of latexdiff with PowerShell but might have some other uses (though I struggle currently to see which ones). Maybe it is possible to internally redirect standard output to designated file within Perl. If this is not possible then every print statement would have to be adapted, which indeed would be a pain.