Feature: Add parameter to configure custom response function

[RafaelWO]

Description

This PR adds the feature of defining custom response functions to process the response, i.e. callbacks. This means that the user can decide what to do with the response.

A simple example might be getting the HTTP status codes for a bunch of URLs:

import asyncio

from unparallel import up

async def main():
    urls = [
        "https://www.example.com",
        "https://duckduckgo.com/",
        "https://github.com"
    ]
    return await up(urls, method="HEAD", response_fn=lambda x: x.status_code)

results = asyncio.run(main())
print(results)

This prints:

Making async requests: 100%|███████████| 3/3 [00:00<00:00,  4.43it/s]
[200, 200, 200]

Main Changes:

• ✨ Add parameter to configure custom response function
• ✅ Add corresponding tests
• 📝 Add documentation about custom response callback

Additional Changes:

• ✨ Add option to disable raising for status
• 🐛 Fix flatten results if there are errors
• ♻️ Refactor script for wordpress
• ♻️ Refactor example for mulitple websites
• 📝 Extends usage docs with section about "other HTTP methods"
• 📝 Add example for requesting multiple websites

Related Issue

Closes #100

Type of Change

• [x] 📚 Examples / docs / tutorials / dependencies update
• [x] 🔧 Bug fix (non-breaking change which fixes an issue)
• [ ] 🥂 Improvement (non-breaking change which improves an existing feature)
• [x] 🚀 New feature (non-breaking change which adds functionality)
• [ ] 💥 Breaking change (fix or feature that would cause existing functionality to change)
• [ ] 🔐 Security fix
• [ ] 🔖 Release

Checklist

• [x] I've read the CODE_OF_CONDUCT.md document.
• [x] I've read the CONTRIBUTING.md guide.
• [x] I've updated the code style using make codestyle.
• [x] I've written tests for all new methods and classes that I created.
• [x] I've written the docstring in Google format for all the methods and classes that I used.