Set pre-release version

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>
Add support for external plugins (located in ext-plugins subdir)

Alternatively, in development mode, it is useful to override it using
{PREFIX}_EXTPLUGINS env. variable.

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>
Extend external plugins support: user provided dirs

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>