Auto-complete for Didact in Markdown files

To help you set up Didact tutorials (*.didact.md, *.didact.adoc), we’ve added a number of auto-completions (accessible by pressing Ctrl+Space while editing an AsciiDoc or Markdown file) to provide most of the basic things you’ll need.

Included completions:

• didact - inserts text for a sample Didact file (# Your First Didact Tutorial)
• Insert Didact Badge - adds a Powered by Didact image with a link back to the github project

• Insert Didact Requirements label - inserts a label that can be updated with Didact requirements testing commands (such as *Status: unknown*{#requirement-name})
• Insert link to install required VS Code extension - inserts a quick link to install a particular VSCode extension ([Click here to install the ExtensionPackName.](vscode:extension/ExtensionPackID))
• Insert Validate All button - adds an HTML button with a link to the Didact validate all command (<a href='didact://?commandId=vscode.didact.validateAllRequirements' title='Validate all requirements!'><button>Validate all Requirements at Once!</button></a>)
• From inside the link target ([link text](link target) for Markdown or (link text)[link target] for AsciiDoc), Start a new didact link inserts didact://? and then offers additional completions

From a didact://? start inside a Markdown or AsciiDoc link, you also get access to every command available in VS Code.