add configuration files for user preferences

[mfrasca]

in the discussion on issue #13 the need came to light to have a configuration file.
it could be named /etc/txt2epub.conf and ~/.txt2epub.conf
on MSWindows systems this would probably not work and I am not so sure about MacOSX.
in @hupili words: »Daily users like you should have already read the docs so they can configure the defaults they want. Other casual users could get a sensible default that works well on a trial doc.«