Allow redefinition of the NewRunner function in RuleSet

[wata727]

Some rulesets redefine Check() to support custom runners in rulesets. e.g.
https://github.com/terraform-linters/tflint-ruleset-aws/blob/v0.16.1/aws/ruleset.go#L46-L59
https://github.com/terraform-linters/tflint-ruleset-google/blob/v0.19.0/google/ruleset.go#L46-L59

What we want to do here is inject a custom runner, so it's nonsense to redefine checking.

Define a NewRunner function in RuleSet so that built-in rulesets always use that function. This allows custom rulesets to inject a custom runner by simply redefining the NewRunner(). No need to redefine Check().