Highlight html custom tags in markdown files

[mjbvz]

VS Code side issue for microsoft/vscode-markdown-tm-grammar#53

[mjbvz]

Fixed by bfda914

[isidorn]

Verification steps please

[mjbvz]

In a markdown file, try using non-standard html tags. For example:

# header

<Graph />
<cat type="tabby"></cat>

These should now be highlighted