cfnLint.path doesn't resolve ${workspaceFolder}

Hi,
I was having a hard time getting this extension to work until I found https://github.com/awslabs/aws-cfn-lint-visual-studio-code/issues/21.

```
> Determined this file is a CloudFormation Template. file:///Users/user/test/test.yml. Found the string AWSTemplateFormatVersion
> running............. ${workspaceFolder}/venv/bin/cfn-lint --format,json,--template,/Users/user/test/test.yml
> Unable to start cfn-lint (Error: spawn ${workspaceFolder}/venv/bin/cfn-lint ENOENT). Is cfn-lint installed correctly?
```

After doing a `which cfn-lint`, grabbing the results, and making that the value of `cfnLint.Path, things started working.

```
Determined this file is a CloudFormation Template. file:///Users/user/test/test.yml. Found the string AWSTemplateFormatVersion
running............. /Users/user/test/venv/bin/cfn-lint --format,json,--template,/Users/user/test/test.yml
```

Looks like `${workspaceFolder}` isn't being resolved. Is this the expected behavior?

Reproduce the failure with cfnLint.path set to ${workspaceFolder}/venv/bin/cfn-lint, then trace how that setting reaches the process spawn. Done means the workspaceFolder variable resolves to the workspace path before cfn-lint runs, while the existing absolute-path configuration continues to work.

Written by the indexing model from the issue text.