Add GetOriginalwd method

[wata727]

Fixes #218
See also terraform-linters/tflint#1612
See also terraform-linters/tflint#1622

This PR adds a new gRPC method called GetOriginalwd(). This method can be called from the Runner and can get the original working directory of the host process (TFLint).

Normally this is equal to os.Getwd(), but differs if --chdir or --recursive is used. If you need the absolute path of the file, joining with the original working directory is appropriate.

$ tree
work
└── sub
    └── main.tf

1 directory, 1 file

$ pwd
/work
$ tflint --chdir=sub

os.Getwd() // => /work/sub
runner.GetOriginalwd() // => /work
runner.GetFile(path).Body.MissingItemRange().Filename // => sub/main.tf