mailer: Make delivery_method configurable

`delivery_method` is set to `smtp` by default. With this patch, we'll be able to change it to `sendmail` like this:

```javascript
{
  "mailer": {
    "mail_from": "sensu@example.com",
    "mail_to": "monitor@example.com",
    "delivery_method": "sendmail"
  }
}
```