Add option to filter out some files from the appcache

You will occasionally have files in the project (like CNAME, iOS splash screens, and manifest.webapp) which you do not want cached.  It would be nice if there was an 'exclude' or 'filter' option which allowed filtering out some of the files.

One option would be for 'exclude' to take a regexp.  A more flexible option might be for a 'filter' option taking a function.
