Insert additional separators to the HTML title if the 1st level heading contains raw line breaks `<br>`

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## Rationale
It is possible to insert line breaks in 1st level headings by exploiting the `
` tag, however this generates ugly HTML title as the line break isn't represented. Please support inserting additional separators like ` `, ` - ` or ` | ` where the line breaks are available to make it less uglier.

## Bonus
Allow user to decide the behavior and separator pattern via note metadata.

## Additional Information
This also includes Book Mode and Slide Mode, which is HackMD EE features.

Start by locating the code that derives the HTML title from the first-level heading and check how raw
line breaks are represented. Compare the behavior in normal, Book Mode, and Slide Mode notes; done means line breaks produce the intended title separators and any metadata-based configuration has a defined behavior.

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