Direct conversion from markdown to html with hackmd-cli

Hi, I am looking to integrate an auto-publishing pipeline with GitHub Actions. Our current documentation is written in HackMD-favoured markdown, is there a way to directly convert markdown to html file with a command like the following:

```
hackmd-cli export --import filename.md --html filename.html
```

Many thanks!

Start by tracing the CLI command surface around the proposed `hackmd-cli export --import filename.md --html filename.html` invocation and how HackMD-favoured Markdown is handled. Done means a Markdown file can be converted directly to an HTML file through the command, including use in a GitHub Actions publishing pipeline.

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