Support linking to interlinks objects.inv

When a link is unresolved, pydoctor could look into sphinx object inventory mapping and try to get a valid link.

intersphinx_mapping = {
      'Crypto' : ('http://py.crypto.com/api', None),
      'twisted: ('http://tm.tld/documentation/current/api', 'index.inv'),
      }

For unresolved L{Crypto.something} it will check the http://py.crypto.com/api/objects.inv file

For unresolved L{twisted.something} it will check the http://tm.tld/documentation/current/api/index.inv file

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Since pydoctor only has simple configuration file... and does not support python code for configuration... the intershinx configuration could look like

--intersphinx Crypto:http://py.crypto.com/api/objects.inv
--intersphinx twisted:http://tm.tld/docs/api/index.inv