ContentService - fix create_node() avoid a hard-coded reference to the Node class in plugins

See:
  https://github.com/dradis/dradis-nessus/commit/0714b94f6474d2dbafe9563d1cedece818db6363
ContentService - reuse the plugin.output node if exists
Add basic Export plugin class
Add Templates module

The Dradis::Plugins::Templates module allows applications to copy any
templates provided by the implementing plugins into a local folder. This
is typically done in an initializer or a first-run process that copies
the default templates from the plugin source and into a local
./templates/ folder where they can be customised by the user.
Bump version to 3.0.0