tflint: Add WalkExpressions function

[wata727]

This PR adds WalkExpressions function as a new runner API. This function is a wrapper that applies a callback function to all hcl.Expression retrieved using GetFiles().

This function is useful for detecting all variable references contained within expressions, or forbidding certain resource references entirely. The argument can be a simple callback function, or any struct that satisfies the ExprWalker interface. If you pass a callback function, it will be invoked for every expression.

The internal implementation uses hclsyntax.Walk. So for example a callback will be called 3 times to "${var.foo}-${var.bar}" for itself, var.foo and var.bar. On the other hand, note that it behaves differently for JSON syntax. A callback is not called for var.foo and var.bar. Nor does it recursively traverse maps or objects. This is an interface issue for the JSON syntax. Please see hashicorp/hcl#543 for details.