added the ability to specify 'builtin' directories in rpm task

- previously, there was a static set of 'builtin' directory names
  (/etc, /opt/, ...)
- this didn't support cases where the RPM content was being deployed
  to an existing directory that didn't happen to be in the builtin list
  (the most obvious case being /etc/rc.d/init.d on RedHat/CentOS)
Merge pull request #54 from bpd/parameterize-builtins

added the ability to specify 'builtin' directories in rpm task