Allow automatic downgrading

This pull request adds a new field to props.plist which allows to define a list of
package version which this package will revert. xbps will install this package if the installed
version matches one of the version of the revert-list even if the installed version is higher.

This allows to roll back certian broken packages to an older version without any user interaction.
Please update the xbps-rindex.8 manpage.