Add async support

[encryptize]

In some cases, scraping speed is of significant importance, so I decided to create a PR with the addition of an async version of this project. What remained was to update the tests and add a mention of async support in the README.

Most important changes:

• Optimization of the function responsible for randomizing the useragent
• A change of the HTTP client from requests to httpx. This change is to use one common library for both sync and async versions.
• Create a shared base class for wrappers.

[Giglium]

Hi @encryptize,
Thanks for the amazing work you have done! In the next few days, I will adjust the test and review the PR more thoroughly. Great catch on optimizing the randomizing user agent!

Is it not discouraged to re-create the client every time? Shouldn't we create it in the constructor and reuse it every time?