Expose ammonia's url_relative policy via url_relative kwarg

[gghez]

Closes #129.

Adds a url_relative keyword argument to clean() and Cleaner() mapping to
ammonia's UrlRelative enum, so relative URLs in href / src /
<object data=...> can be passed through (default), denied, rewritten against a
base or root URL, or rewritten by a custom callback.

API

Python value	ammonia
None (default) / "pass_through"	PassThrough
"deny"	Deny
("rewrite_with_base", base_url)	RewriteWithBase
("rewrite_with_root", root_url, path)	RewriteWithRoot
`Callable[[str], str	None]`

nh3.clean('<a href="/foo">x</a>', url_relative="deny")
# '<a rel="noopener noreferrer">x</a>'

nh3.clean('<a href="/foo">x</a>', url_relative=("rewrite_with_base", "https://example.com"))
# '<a href="https://example.com/foo" rel="noopener noreferrer">x</a>'

nh3.clean('<img src="/a.png">', url_relative=lambda url: f"https://cdn.example.com{url}")
# '<img src="https://cdn.example.com/a.png">'

Notes

• Default is unchanged (None ⇒ ammonia's PassThrough); no behaviour change for existing callers.
• Input is validated eagerly at construction: ValueError for an unknown mode
  string, malformed tuple, or unparseable URL; TypeError for an unsupported type.
• A custom callback that raises (or returns a non-str/non-None value) strips
  the URL and reports the error via sys.unraisablehook, keeping clean()
  infallible — mirroring attribute_filter's error handling.

Tests

Adds 9 tests (deny / rewrite_with_base / rewrite_with_root / custom replace /
custom strip / custom raising / explicit pass_through / invalid inputs /
reusable Cleaner), plus doctest examples on clean. nh3.pyi stubs updated.