Allow customized copyright contents

[shenchsh]

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:

What is the current behavior?

This theme does not support customized copyright contents.

What is the new behavior?

Use can defined there copyright contents in post

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

How to use?

In post, user can use copyright in front-matter block to define their own copyright contents

Does this PR introduce a breaking change?

• Yes.
• No.

[sli1989]

Can u provide the screenshots of this features in all schemes?

[ivan-nginx]

No need this. Custom language feature (#253) already suppport custom translates for all keys. Docs will be later.