The plugin $:/plugins/tobibeer/join provides join[]:
Syntax
[join[<item-template>\<option>:<option-value>\<option>:<option-value>]]
Specify an item template for the join[] filter, as well as further options as key:value.
[join[]] — simply join input titles[join[<pre>%title%<post>]] — join via item template[join[sep:- ]] — join with separator - [join[trim:result]] — trim end result[join[trim:each]] — trim each input title[join[prefix:+]] — final prefix[join[suffix:+]] — final suffix[join[pre-%tile%\trim:each\prefix:ok:\sep:+]] — combine options