Moved caption default into body of macro

Found in `gae-init-babel` (yes, downstream) that trying to use a `caption=_('Next Page')` default parameter on the `next_link` macro doesn't work: it throws a `NameError: global name 'l__' is not defined`

Probably due to something in Jinja2 parsing the macro definition.

Nevertheless, that is why I propose to remove the parameter default on the macro definition and use an inline if-else in the macro body instead. That will allow `gae-init-babel` and derived projects to translate it.