Experimental plugin support

[wata727]

Fixes #499
Closes #255

This pull request introduces an experimental plugin system. TFLint adopts https://golang.org/pkg/plugin to achieve the goal.

Plugins are placed in the ~/.tflint.d/plugins directory with the naming convention tflint-ruleset-<NAME>.so. You can explicitly enable the plugin by .tflint.hcl as follows:

plugin "NAME" {
    enabled = true
}

That's all. Now you can freely add custom rules to TFLint!

Plugins are provided as a single *.so file and can be easily built with go build --buildmode=plugin main.go. You can see an example: https://github.com/terraform-linters/tflint-ruleset-template

The plugin must satisfy the following constraints:

• Name function is implemented under the main package.
  • It should return the plugin name as a string.
• Version function is implemented under the main package.
  • It should return the plugin version as a string.
• NewRules function is implemented under the main package.
  • It should return a list of rules that implement plugin.Rule interface.
• It is built using the same TFLint version.

Note the plugin package is only supported on Linux and macOS. Therefore, this plugin system does not work on Windows.