Changed class variables to instance variables in plugins

This PR changes the class 'hierarchy' variables `@@description` and `@@confidence` to instance variables within each plugin (and supporting code). Before this change, each time a plugin was required by [plugins.rb](https://github.com/rapid7/memorandom/blob/8630da34a5536ed83b8605f5cee577ab3222631b/lib/memorandom/plugins.rb), the variables were found in the class hierarchy (which looked up to the PluginTemplate class). The value of the variables were then overwritten. This caused the list of plugins to only show the description of the last plugin required (and set the confidence of each plugin to show as 0.10).