Introduce autofix

[wata727]

Fixes #266
See also terraform-linters/tflint-plugin-sdk#254
See also terraform-linters/tflint-ruleset-terraform#93

This PR introduces autofix feature. Running tflint --fix will automatically fix issues as possible. Below is an example of autofix in the terraform_comment_syntax rule:

$ cat main.tf
// EC2
resource "aws_instance" "foo" {
  instance_type = "t2.micro"
}

$ tflint --fix
1 issue(s) found:

Warning: [Fixed] Single line comments should begin with # (terraform_comment_syntax)

  on main.tf line 1:
   1: // EC2
   2: resource "aws_instance" "foo" {

$ cat main.tf
# EC2
resource "aws_instance" "foo" {
  instance_type = "t2.micro"
}

In order for autofix to work, the rule that emits the issue must support autofix. This should take advantage of the EmitIssueWithFix API added in the latest SDK version.

Technically, rewriting of files by autofix is completed on the plugin side. Once the rewrite is complete, the plugin sends the changed file per rule to the host server (TFLint) and stores the changes in-memory. TFLint finally writes to files when all inspections are completed successfully. See terraform-linters/tflint-plugin-sdk#254 for more information on plugin-to-host server communication and autofixer implementation.

[nitrocode]

Wow!!! Nice work @wata727 ! Thank you

[davenicoll]

@bendrucker can you please create a new release? thanks

[wata727]

I'm planning on adding a few more changes in the next release. The next release will come out in this month.