Accept a directory as an argument

[wata727]

Fixes #296

Currently, only files under the current directory are inspected, and an error occurs when passing a directory as an argument like the following:

$ tflint tests
Error: Failed to load `tests`: TFLint doesn't accept directories as arguments

Initially, this behavior was implemented as a respect to the Terraform behavior, but Terraform can accept a directory of configuration files as an argument. See https://www.terraform.io/docs/commands/plan.html

Based on the above, this PR makes TFLint accept a directory as an argument. However, as with Terraform, you cannot pass multiple directories.

$ tflint example tests
Error: Failed to load `example`: Multiple arguments are not allowed when passing a directory

Additionally, It also allows files not in the current directory to be accepted. In that case, the directory of the passed files is loaded, and only the issues of the passed files are reported. Therefore, please note that you cannot pass files in different directories at the same time.

$ tflint example/*.tf
Awesome! Your code is following the best practices :)
$ tflint example/*.tf terraform/*.tf
Error: Failed to load `terraform/cloudfront.tf`: Multiple files in different directories are not allowed