Smarter external relative links

[DavidLeoni]

External relative links to projects in the same org should be rewritten to take into account other projects having same josman structure

Example from josman-maven-plugin/blob/master/TheTests.md:

../../../jackan/blob/branch-0.3/README.md

Should be rewritten to

../jackan/0.3/index.html ../../../jackan/0.3/README.md

So to pick http://opendatatrentino.github.io/jackan/0.3/index.html

To avoid hardcoding versions, this plugin looks interesting. BUT in order for it to work we should make Maven variable injection work, see #3

[DavidLeoni]

This thing was harder to implement than expected, will defer it to versions > 0.8 ....