[plugin] Fix use of internet resources against local target #37

These commits provide a boolean switch for a plugin in order to fix the issue #37

If that switch is set to True and if the plugin is run against a local target, then OWTF skips the current plugin and displays the message: *"Skipped - Avoid use of internet resources against local target"*

If you have any suggestions about this fix, please tell me. Especially about the skip part (for instance, should we specify a *Status* like when an user is aborting a plugin? @tunnelshade @7a)

The current limitation concerns the local IPv6 detection that does not exist yet.

*(The first commit only concerns pep8 indentations)*