Add reading_progress to vendors

[maple3142]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines.
• Tests for the changes have been added (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs have been added / updated (for bug fixes / features).

PR Type

What kind of change does this PR introduce?

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Build related changes.
• CI related changes.
• Documentation content changes.
• Other... Please describe: add field in _config.yml

What is the current behavior?

User doesn't know reading_progress can use cdn.

Issue Number(s): #105

What is the new behavior?

User knows reading_progress can use cdn.

• Screens with this changes: N/A
• Link to demo site with this changes: N/A

How to use?

In NexT _config.yml:

vendors:
  # something....
  reading_progress: #cdn url

Does this PR introduce a breaking change?

• Yes.
• No.

[wafer-li]

Use min.js as the example.

[wafer-li]

Use min.js as the example will be better I think.