Expanded behavior for tilt

I propose an extension to the tilt exe, as outlined in the usage information.  I think this will make tilt much more capable because you could render a whole series of templates from an input dir to an output dir.  

I plan on building this behavior somewhere.  If you like this proposal, then I'll write it into tilt itself.  Otherwise, I'll make a separate fork or project.

Thanks!  Appreciate the great library!

Use Case
-----------------

    mkdir -p inputs outputs
    echo "obj: milk" > attributes.yml
    echo "Got <%= obj %>" > inputs/file.txt.erb
    tilt --attrs attributes.yml --input-dir inputs --output-dir outputs --files inputs/*

The result would be 'outputs/file.txt' with content 'Got milk'.