Add GetModulePath method

[wata727]

Fixes #165

This PR adds a new gRPC method called GetModulePath. This method can be called from the Runner and can be used to determine where the current module is in the module tree.

For example, IsRoot() can be used to determine whether a module is a root module:

path, _ := runner.GetModulePath()
if path.IsRoot() {
  // In the root module
}

[yashg-ti]

Hi
To be able to use this api in tflint-ruleset's check function, what's the required way of using the tflint command on the root file? I'm assuming it takes some recursive approach of progressing through the terraform tree. If so how to make this recursion happen?

I'm using the iterative approach currently, where I run the tflint command individually on each child module, thinking it picks the path up from the module path parameter in the command, which is kept relative to the root file directory. But that doesn't seem to be the case as it's not working, and returning 'nil' for child modules.