Custom theme support

[yhatt]

Resolves #39. I added markdown.marp.themes configuration property to specify paths for custom theme CSS. It has the same rule as VS code built-in configuration markdown.styles: Specified paths must be URLs and relative paths from workspace directory.

Marp requires the actual content of files unlike Markdown preview. Thus, we have to add the logic of resolving themes from URL or workspace/parent directory of Markdown to support Marp's custom theme in preview.

Resolving paths looks like difficult because we cannot access to the original document of VS Code in the context of markdown-it extension normally. But I found a trick that using normalizeLink method defined to markdown-it instance can resolve its workspace directory or parent directory. We use this trick to get same behavior of markdown.styles.

Advantage of markdown.marp.themes is accepting external URL, very easy way to get community-driven CSS themes.

// Use custom theme by URL
{
  "markdown.marp.themes": ["https://raw.githubusercontent.com/yhatt/marp-cli-example/master/themes/example.css"]
}

ToDo

• Allow using custom theme CSS while exporting
• Add tests

[yhatt]

CI was passed by filled unit tests.