feat: allow user to provide custom pseudo elements to be captured

[SUCHMOKUO]

Hi! Thanks for maintaining this great library!

I've tried it and just found that it might be great if we can control what pseudo elements will be included in the captured image rather than just ::before and ::after, sometimes we might want to include some non-standard but useful ones, like ::-webkit-scrollbar and ::-webkit-scrollbar-thumb, so I created this PR to add a new option includedPseudoElements to allow user to customize the rule of the included pseudo elements.

Hope this can be helpful :)

Description

1. Add new option includedPseudoElements.
2. Refactor clonePseudoElements to receive the includedPseudoElements list and clone all the pseudo elements in the list.
3. Add test for this option.
4. Update README.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Enhancement (changes that improvement of current feature or performance)
• Refactoring (changes that neither fixes a bug nor adds a feature)
• Test Case (changes that add missing tests or correct existing tests)
• Code style optimization (changes that do not affect the meaning of the code)
• Docs (changes that only update documentation)
• Chore (changes that don't modify src or test files)

Self Check before Merge

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[biiibooo]

👋 @SUCHMOKUO

💖 Thanks for opening this pull request! 💖

Please follow the contributing guidelines. And we use semantic commit messages to streamline the release process.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.93%. Comparing base (b751cbf) to head (f6363fc).
Report is 13 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #376   +/-   ##
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  Coverage   62.93%   62.93%           
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  Files          10       10           
  Lines         580      580           
  Branches      143      143           
=======================================
  Hits          365      365           
  Misses        151      151           
  Partials       64       64           

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