avocado.plugins: Turn the lister into a core plugin

The lister plugin is useful, as it allows people
to see present, active/inactive plugins. So split
it from the examples area.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
examples: Move avocado hello to examples/plugins and rename

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #19 from avocado-framework/split-example-plugins

Split example plugins