Extended syntax for SSI: a custom function can be passed

In the Compara documentation, there are several modules that are very very short. They are included in the HTML pages with a [[SCRIPT::] tag, and basically only define the name of a Perl method that will do the rendering. This is because the default SCRIPT tag is supposed to call render()

This PR allows to append the name of a function, and some arguments, that should be called instead of render(). It simplifies a lot the Compara documentation by allowing to directly call the methods that do the rendering, and 7 files could be removed. This PR is linked to Ensembl/public-plugins#11, which updates the HTML files to call the right methods.

Matthieu