I found this plugin named grunt-includes. I've been looking for an answer to your question forever. This is the first one I've seen that seems easy to use. All the others seem to maybe have this feature but do 20x other things so they seem like the wrong tool for the job.
The one thing this is missing for me is the ability to prefix relative paths. I'm talking to the developer of modest which is becoming a better option.
UPDATE: There's a grunt-includes-replace that is almost as simple and can prefix relative paths. IE: it lets you pass in variables.
