MODULES-444-Add concat multiple

There was a request to enable concat to take multiple arguments.

Wasn't sure people wanted to change concat, so I added a new function. I can refactor if there's a consensus that we can change the original concat to accept > 2 arguments :+1: 

https://tickets.puppetlabs.com/browse/MODULES-444
I went for a soflty approach at first, because I wasn't sure how PL was dealing with backwards compatibility on new features on functions.

If it's cool to change so concat could take > 2 arguments I can definitely refactor this PR to be on the original function and DRY up the code a bit :+1: