Main: Remove obsolete instance variable
Main: Make workdir persistent and configurable

Allow loading workdir from command line, but afterwards, store it into ebook
configuration file and read it from there.

The command line option can be used anytime to override configured workdir and
start ebook processing from fresh start.
Main: Move Datafile class into separate file

Move unit tests and definition of EBookize::Chapter::Datafile class into
separate .rb file so that the Chapter class is easier to read. This will become
even more important as we add more nested classes.
Main: Add logging of progress

Use Logger class to output messages with different severities into stdout. Add
log messages into Application and Chapter classes.