Make the slug configurable

[agonzalezro]

It's possible that some user wants to migrate from a non-default slug configuration in Pelican.

If they do that we are not going to be able to offer a nice retrocompatibility because:

• a lot of pages are going to 404 since we just support one slug.
• Disqus is not goint to show the old comments because we use the slug there.
• ... -write some odd behaviour here-

Please, fix.