Giveaway Description Message

**Is your feature request related to a problem? Please describe.**
Right now there isn't a good option on the [messages option object](https://discord-giveaways.js.org/global.html#GiveawayMessages) to specify a description on the giveaway without over-riding the 'prize' field. And over-riding prize gives you confusing messaging if you then try to output the prize as a part of the win message.

As an example if I wanted my giveaway embed to be formatted like:

```
The GenericDiscordServer Team is giving away a month of discord nitro as a part of our ongoing anniversary event!
{inviteToParticipate, etc}
```
It would mean setting prize to that message and the win message would look something like:
```
Congrats @/user ! You've won **The GenericDiscordServer Team is giving away a month of discord nitro as a part of our ongoing anniversary event!**
```

Which is confusing.

**Describe the solution you'd like**

A 'description' message option that can be specified like:
`The GenericDiscordServer Team is giving away {prize} as a part of our ongoing anniversary event!`

and would replace where `prize` would normally go by itself in the main embed.

**Describe alternatives you've considered**

I thought about over-riding the 'inviteToParticipate' message with additional content, but since that would still have the title of the embed with just the prize, that would look pretty strange too. I'm also considering overwriting the `generateEmbed` functions with something custom myself to provide myself with the solution I described above, but obviously it'd be nice if it was included in this package.