Prints deprecation warnings for deprecated options
Create the input (string or file path)
Returns true if the file given as self.input is a Markdown file.
Extracts --extern-html-root-url arguments from matches and returns a map of crate names to
the given URLs. If an --extern-html-root-url argument was ill-formed, returns an error
describing the issue.
Reports error if βinclude-parts-dir / crate-info is not a file
Extracts read_rendered_cci and write_rendered_cci from command line arguments, or
reports an error if an invalid option was provided