Add configuration / parameters for join_lines command

What would be nice is if the join_lines command took an argument. If you don't pass anything, it behaves as usual. If you pass a string in, that'll be what separates the lines. If I pass ", " then I'll get a comma-and-space delimited list. If I pass "" then there won't be any spaces between the lines. If I pass " " then I'll get the default behavior - a single space between the lines.

Along with that, it'd be nice to have a configuration option where I can override the default behavior. Some setting where I can store the default parameter for the command. Unspecified, it's " " - but I could specify "" to override and always have no space between lines.