Support to manual configure the sidebar width of the Gemini Scheme.

[tsanie]

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. (not affected)
  • 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?

Related Issue Number(s): #67, iissnan/hexo-theme-next#1980

What is the new behavior?

Allow the users configure their sidebar width for Gemini Scheme manually.

• Screens with this changes: N/A
• Link to demo site with this changes: https://tsanie.us (with a custom sidebar width: 300px)

How to use?

In NexT _config.yml:

# Manual define the sidebar width
# !!Only available for Gemini Scheme currently
# Leave it empty for the default 240
sidebar_width: 300

Does this PR introduce a breaking change?

• Yes.
• No.