Required groups

Might be interesting to support something like

```go
type Conf struct {
    ThisThing string    `conf:"requireOne:things"`
    OrThisThing string  `conf:"requireOne:things"`
    SomeNumber int      `conf:"requireOne:numbers"`
    OrAnotherNumber int `conf:"requireOne:numbers"`
}
```
Allowing the user to specify that at least one of a particular group of options is required.