Extract edition from a command instead of docinfo #17

Now you can pass the edition as paramater:

a2x -f pdf -a edition=2 ...

or you could use git describe:

a2x -f pdf -a edition=`git describe`

Remeber: if you are setting the edition inside `docinfo.xml` you will have to remove from there.

If you have specified two editions tags docbook uses the last one defined.

This solution will use the value used from the param instead of the docinfo xml.

Resolves #17