Add add_*/rm_* keyword arguments for set-valued sanitizer options

[gghez]

Closes #78.

Summary

Exposes ammonia's incremental builder methods alongside the existing
replacement options on both nh3.clean() and nh3.Cleaner:

• add_tags / rm_tags
• add_clean_content_tags / rm_clean_content_tags
• add_url_schemes / rm_url_schemes
• add_generic_attribute_prefixes / rm_generic_attribute_prefixes

Each new option applies on top of the corresponding replacement option
(or the ammonia defaults when it is omitted), so a caller can extend the
default whitelist without first copying it:

# Before:
nh3.clean(html, tags=nh3.ALLOWED_TAGS | {"my-tag"})

# After:
nh3.clean(html, add_tags={"my-tag"})

rm_* applies last, so it wins on conflict with add_* — mirroring
the ammonia builder's ordering.

Notes

• The filter_style_properties option that the issue also requested
  has already shipped (it is in v0.3.x), so this PR covers only the
  remaining add_tag-style request, generalized to the four
  set-valued options that ammonia exposes add_*/rm_* for.
• The existing clean_content_tags overlap guard (PR Validate clean_content_tags conflict with tags #125) now
  computes the effective tag sets after add_*/rm_* are applied,
  so e.g. add_tags={"p"} plus add_clean_content_tags={"p"} raises
  ValueError rather than panicking ammonia.
• Mapping-shaped options (attributes, allowed_classes,
  tag_attribute_values, set_tag_attribute_values) keep the
  replacement-only behaviour — designing an ergonomic additive API
  for nested dicts is out of scope here.

Test plan

• maturin develop --release builds cleanly
• python -m pytest -v — 23 passed (8 new tests + existing 14 + 1 RST doctest)
• cargo fmt --check — silent
• cargo check — exit 0
• Manually verified the new RST doctest in docs/index.rst runs as part of the suite

[Copilot]

Pull request overview

This PR extends nh3’s Python API to support ammonia’s incremental “builder” modifiers (add_* / rm_*) for set-valued sanitizer options, allowing callers to extend or subtract from defaults without copying full whitelists.

Changes:

• Added add_* / rm_* keyword arguments for tags, clean_content_tags, url_schemes, and generic_attribute_prefixes to both nh3.clean() and nh3.Cleaner.
• Updated overlap validation between allowed tags and clean-content tags to consider the effective sets after modifiers are applied.
• Added tests, type stubs, and documentation covering the new options and examples.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/lib.rs	Adds new config fields, wires the new keyword args through to ammonia builder methods, and updates overlap validation logic.
tests/test_nh3.py	Adds new pytest coverage for add_* / rm_* behaviors and modifier interactions.
nh3.pyi	Updates public type signatures to include the new keyword arguments.
docs/index.rst	Documents how to extend/remove from defaults using add_* / rm_* options with doctest examples.

Comments suppressed due to low confidence (1)

src/lib.rs:456

• The ValueError message references only tags and clean_content_tags, but conflicts can now be introduced via add_tags/rm_tags and add_clean_content_tags/rm_clean_content_tags as well. Consider updating the message to describe the effective allowed vs clean-content sets and suggest resolving it by either removing from the clean-content set or removing from the allowed set (including via rm_tags / rm_clean_content_tags), so callers know how to fix conflicts introduced by the new modifiers.

            if let Some(tag) = effective_clean.intersection(&effective_allowed).next() {
                return Err(PyValueError::new_err(format!(
                    "tag \"{}\" cannot appear in both `tags` and `clean_content_tags`; \
                     either remove it from `clean_content_tags` or pass an explicit \
                     `tags` set that excludes it",
                    tag
                )));

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