Use named arguments in `slim_block` to add templating support to non-code arguments.

[rdinnager]

It would look like this:

slim_block(1, ..end_gen.. = 1000, { code_goes_here }). So the same rules apply as for a normal slim_block() call, but if one of the arguments is named, the name is interpreted as a templated variable name, and the argument value is the default value for that templated variable. Require the ..var.. pattern for consistency with slimr_template(). The end_gen templated variable could then be replaced in a slim_script_render() call with a list element named "end_gen".

[rdinnager]

I've now used named arguments to support multispecies models in SLiM >4.0, but this could still work by testing for the ..var.. construction, assuming no one will want to name their species ..something..