zshrc: Add function cdt() to create a temporary directory and change current working...

Thanks to Bernhard Tittelbach <bernhard@tittelbach.org>
for the suggestion, thanks to Julius Plenz <zshrc@plenz.com>
for his initial idea at http://www.plenz.com/tmp/setup/zshrc
and to Carsten Hey <carsten@debian.org> for the idea naming
it 'cdt' (instead of cdtemp).