Added a "dice" plugin.

This is a plugin that allows you to emulate dice rolling (or just random number generation), like so:

```
|dice 10d6
```
Will generate 10 rolls of 6-sided die

```
|dice 20d1000
```
Will generate the total of 20 random numbers from 1-1000.

```
|dice 1000d20
```
Will generate the total of 1000 random numbers from 1-20.