Fix site title's external font family bug

[leirock]

PR Checklist

Please check if your PR fulfills the following requirements:

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

What is the current behavior?

When we set external font family of site title, no change happens.

font:
  enable: true
  title:
    external: true
    family: Caveat
    size:

Issue resolved: N/A

What is the new behavior?

We can set external font family for site title.

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

How to use?

In NexT _config.yml:

font:
  enable: true
  title:
    external: true
    family: Caveat
    size:

Does this PR introduce a breaking change?

• Yes.
• No.

[1v9]

@allcontributorsadd please add @lei2rock for doc, bug

[allcontributors]

@1v9

I've put up a pull request to add @lei2rock! 🎉