Added a "dice" plugin.

This is a plugin that allows you to emulate dice rolling (or just random number generation), like so:

```
|dice 6 10
```
Will generate 10 rolls of 6-sided die

```
|dice 1000 20
```
Will generate the total of 20 random numbers from 1-1000.

```
|dice 20 1000
```
Will generate the total of 1000 random numbers from 1-20.