Fix #35: HTML, Math, and Markdown in headings are supported

[ken1kob]

This PR fixes issue #35 and provides the following features:

1. Headings in the Outline panel are consistent with ones in Viewer as possible.
2. In Viewer Layout mode, clicking such headings works correctly.

Supported annotations are:

• (nested) HTML tags
• (multiple) math expressions ($ ... $)
• (multiple) Markdown links
• (nested) Markdown emphasis and strong (* and _)

Test cases: test_headings.md

[cqroot]

I will test this feature.

[cqroot]

@all-contributors please add @ken1kob for code.

[allcontributors[bot]]

@cqroot

I've put up a pull request to add @ken1kob! :tada: