• VSCode Version: 1.21.1
• OS Version: MacOS High Sierra 10.13.3 (17D102)

Steps to Reproduce:

1. Using markdown preview in combination with plantuml renderer
2. run local plantuml server docker run -d -p 8088:8080 plantuml/plantuml-server:tomcat
3. configure code to use local server for rendering "plantuml.server": "http://localhost:8080",
4. preview a markdown with embedded plantUML content
5. get error about unsafe content

The current code security settings for md preview only allow to enable/disable unsafe (HTTP without the S) entirely. Would it make sense to add an option that allows unsafe content from localhost but not from remote sources?