Move Plugins to seperate repos

@ttscoff, this was my first attempt at moving over the plugins. This is in reference to https://github.com/ttscoff/Slogger/issues/271

I have set them up in repos here: https://github.com/sloggerplugins and I'd be happy to transfer that org over to you. I also have a quick script to create the repos on github and upload them if you'd prefer to just keep them on your own account. I have no opinion I just needed something to get it working!

At this point you can install a plugin using `rake plugin:install[feed]`. You can also `rake plugin:enable[feed]` or `rake plugin:disable[feed]`. 

The plugins and plugins_disabled directories are ignored. The rake task will create a slogger_plugins directory at the same level as the slogger directory, git clone down the plugin and then symlink over the plugin. This should make it easy for users to modify or update individual plugins easily. We could also easily add a `plugins:update[feed]` and `plugins:update[all]` command to make upgrading those easier.


Would love to get any initial feedback on this implementation.