Support repeated annotations

[Zardoz89]

Actually, having the same annotation repeated (ie two @author for example) override the previous. It should list the repeated values.

For example :

<#--
    @author Foo
    @author Baz
-->

Only shows Author Baz.

This happens, because when finds a @ annotation, stored it directly on the output map. Instead, should store it on a list (and the formatting templates should expect to be a sequence of values), so could list repeated annotations.