Add ns_goodmail module

This module adds support for a whitelist of email domains that are allowed to register nicks via NickServ.  This allows you say "only emails from @example.com can register nicks on this server".

I also included a help file though I wasn't sure where it should live.

Some instructions on using the module:

```man
In GOODMAIL ADD, you are able to use glob matching to whitelist email
addresses that match a pattern.

Syntax: GOODMAIL ADD <email> [reason]

Examples:
    /msg &nick& GOODMAIL ADD jdhore1@gmail.com bar reason
    /msg &nick& GOODMAIL ADD *@aol.com bar reason
    /msg &nick& GOODMAIL ADD *@*.tk foo reason

GOODMAIL DEL allows you to remove entries from the list of currently
whitelisted email addresses.

Syntax: GOODMAIL DEL <email>

Examples:
    /msg &nick& GOODMAIL DEL foo@bar.com
    /msg &nick& GOODMAIL DEL *@aol.com

GOODMAIL LIST allows you to list the currently whitelisted email
addresses.

Syntax: GOODMAIL LIST

Examples:
    /msg &nick& GOODMAIL LIST
```