Add new SlashCommands plugin

From the README:

## Examples
To use any of the following, just type them into the chat input text area. Note that any commands which are room-specific (`/topic`, `/kick`, etc) will work on/for the current room only.

### Room Management

* `/join room [password]` - Joins the MUC room "room" with an optional password
* `/part` - Leaves the current MUC room
* `/clear` - Clears the scrollback in the current room
* `/topic This will be the new topic` - Sets the topic for the current room to "This will be the new topic"

### Presence

* `/available`
* `/away`
* `/dnd` - Do Not Disturb


This plugin consolidates the functionality in the "join" and "clearchat" plugins, as well as adds some new functionality.