Type-safe input URLs as method arguments whenever possible

[yveszoundi]

Fix issue #5 with the following changes

• Add the url crate dependency for document URL validation
• Add a new DocUrl type to hold pre-validate document URLs (relaxed validation)
• Refactor few existing methods and tests to use the new DocUrl type

[yveszoundi]

I added 2 factory methods for local document URLs construction: local_into_abs and local_as_abs.

• For local files, most of the time people would want not to care about providing either a relative or absolute path.
• One caveat of local_into_abs is that it leverages std::fs::canonicalize, so the file must exists ahead of time