Adds a content attribute to the file module.

There are a lot of cases where a template file is created just to set the
content of a variable to a file. This allows for the same result with a more
direct notation.

Usage example:

    - file: dest=/etc/motd content={{ motd }}

This PR probably needs a bit more work, updating the tests and documentation. I'm submitting this to see if there is support for that feature.