feat: support writing HTML(Vue) anywhere in the header.

[ulivz]

Summary

Since VuePress needs to extract the header from the markdown source file and display it in the sidebar or title (#238), this file(lib/util/parseHeaders.js) simply removes some unnecessary elements to make header displays well at sidebar or title.

But header's parsing in the markdown content is done by the markdown loader based on markdown-it. markdown-it parser will will always keep HTML in headers, so in VuePress, after being parsed by the markdiwn loader, the raw HTML in headers will finally be parsed by Vue-loader. so that we can write HTML/Vue in the header. One exception is the HTML wrapped by <code>(markdown token: '`') tag.

This PR is to support writing HTML(Vue) anywhere in the header, not just at the end of the header.

Some key points about this PR:

1. Write HTML(Vue) anywhere in the header as long as it is not wrapped by code(`).
2. The HTML wrapped by code will be shown as it is.
3. A good practice when using HTML in a header is to leave a space between plain text and HTML.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

Close: #693
Close: #688
Close: #689

A correct solution to replace #694

[ulivz]

Updated docs at 12c2beb