Description of problem:

Currently topic export can be called in two ways:

- With no arguments - assumes user is inside the book, in the directory containing publican.cfg.
- With one argument - assumes user is one level above the book, argument specifies the name of the directory containing the book. This is the original behaviour.

It would be useful to be able to provide multiple arguments, each representing a book within the current directory to export. For instance:

topic export Installation_Guide Administration_Guide

instead of:

topic export Installation_Guide && topic export Administration_Guide