Markdown Highlighting: Unknown Tags

[karlhorky]

Note: Half of this issue was previously opened in #49

The Markdown syntax highlighting grammar doesn't recognize unknown HTML elements (such as web components or components in frameworks):

Compare that with the HTML grammar highlighting, which would be the preferable output:

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Background: In @silvenon's MDX extension, I would like React component highlighting to work properly inline (see silvenon/vscode-mdx#9).

While this wouldn't get us all of the way there, it would at least be a step in the right direction (and would also benefit normal Markdown users).

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Prior art (HTML Textmate Grammar adding custom HTML tags):

textmate/html.tmbundle#92
microsoft/vscode@cfc2a22

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I will submit a pull request with tests for this.