Deprecate AsyncTask. Add RxJava variant of TextCrawler.makePreview()

[dhruvnagarajan]

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Add RxJava variant of makePreview() to TextCrawler
• Refactor
• Move crawler network call and parsing logic to an abstraction
• Move async task implementation to a separate class
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• TextCrawler.extendedTrim(String) requires reference via TextCrawler.Companion.extendedTrim(String), when called from Java.
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules