Add examples of custom variable and function lookups

From the conversation in https://github.com/hashicorp/hil/pull/8

It would be very useful to implement lookup functions for variables and functions without modifying HIL package code.

One example use-case would be reading arbitrary environment variables, eg.

```
greeting = "Hi ${env.NAME}"
```

This initially appeared possible by passing something that implements the [`Scope` interface](https://github.com/hashicorp/hil/blob/master/ast/scope.go#L10-L13) to the [`EvalConfig` function](https://github.com/hashicorp/hil/blob/master/example_func_test.go#L30-L31), however other parts of the code explicitly reference the `BasicScope` implementation making this not possible.

If this is already possible it would be great to document some examples in the README. If not would it be possible to allow other implementations of the `Scope` interface?