NEWS: mention recent changes.
README.md: break lines to fit in 80 cols.
README.md: describe how to run a test.
README.md: fix wrong backtick to end code segment.
bin/xbps-create: allow to define the reverts field for a package.
lib/util.c: add function which tests if a given pkgver is reverted by a given pkg.
bin/xbps-rindex: take the revert field in account.

- if the user wants to add a package which is older than the
  one in the index, xbps-rindex will check if the package
  reverts the one on the index. If so the package will be added
  regardless of its version

- if the user wants to add a package which is newer than the
  one on the index, xbps-rindex will check if the package on the
  index reverts the one the user wants to add. If so the package
  will be skipped regardless of its version.
lib/transaction_ops.c: trans_find_pkg will check the revert field too.

- if a package on the repo has a lower version than installed
  libxbps will check if this package reverts the one installed
  and downgrades to it if so.
rename xbps_pkgver_is_reverted to xbps_pkg_reverts
xbps_pkg_reverts: use version instead of pkgname-version to find reverts
xbps_pkg_reverts: add unit tests to test this functionality.