Allow user direct access to client when initating a plugin

[kyrivanderpoel]

Right now, plugins are initiated only with a plugin configuration. We should give the plugin access to the client created in discordbot.js, so a user can change their plugin code from this:

if (message.author.username != config.client.user.username) {
...

To this:

if (message.author.username != this.client.user.username) {
...