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:

• https://github.com/tcort/markdown-link-check/issues/117
• https://github.com/tcort/markdown-link-check/issues/280
• https://github.com/tcort/markdown-link-check/issues/40 (kind of)