[crawler] option to disable plugins

[tino]

Hi,

It would be nice to have a --disable-plugin option, to disable the time plugin for instance.

I started on a patch, but it is a little difficult at the moment, as the 'default active' plugins are setup in de the Crawler.__init__ method. I think the right solution for this is to do away with the active attribute, and populate the plugins option of the management command with the plugins that should be enabled by default. That way it is easy to disable plugins.

What do you think?