support for revved files with suffix/postfix naming

This is the usemin piece to support grunt-rev's change to suffix/postfix naming described in cbas/grunt-rev#2.  The corresponding grunt-rev PR can be found at cbas/grunt-rev#8.

The necessary changes were larger than I expected, partially due to a desire to get the tests to pass on Windows as well as Mac.  If you'd like to reduce the changes, let me know and I'll  take out the Windows-specific changes.  If it's preferable to wait until v2.0 is merged into master, I can wait for that and then rebase this PR onto master.

I've signed the CLA.