Recursive folders, aliases, parallel requests, simplifications

[pbochynski]

Several features and improvements are introduced in this PR:

• processing folders as inputs (folder is processed recursively)
• support for alias replacement in the URL (used for checking aliased urls to the static site like docsify)
• configurable number of parallel requests
• removed dependencies (needle, link-check) and replaced by native fetch
• several simplifications in the code (e.g. reading files)

Refactoring was done without changing functions signatures and keeping the existing tests.

Resolves:

• Make Async Map threads number configurable #117
• Re-evaluate need for some dependencies? #280
• Add native globbing of files #40 (kind of)

[pbochynski]

Too many changes were required to optimize the code and support fast parallel processing. I had to rewrite it completely.