Prompt variable definitions don't stop at the first newline character

[PladsElsker]

For instance, if I write the following prompt, then the rest of the prompt is used as the variable definition scope, and not just what's defined on the line:

$label = prompt

this is my $label

It works if I add parentheses around it, though (but they are still parsed as weights):

$label = (prompt:1)

this is my $label

[ljleb]

I'll do it along with #16. This will also allow to use variables in places that expect weights or sampler step indices.