adding an optional delimiter argument to the assemble module

Hello, I added an optional "delimiter" argument to the existing assemble module. It was necessary for me to complete the playbook I was working on, so I'm sure it would be useful to someone else out there.

The use case is sticking together bits of json or other things with a common delimiter that can't be included in the files themselves nicely.

You guys are doing a great job, cheers!