fix: wire --regex-pattern through the CLI (#175)

[patchwright]

Fixes #175.

Problem

The CLI defines --regex-pattern in parse_args, but slugify_params() never passes regex_pattern to slugify() — every other argument is wired except that one. So the flag is silently dropped:

$ slugify --regex-pattern "[^-a-z0-9_]+" "___This is a test___"
this-is-a-test          # expected: ___this-is-a-test___

(@peter-bloomfield reported it; @jdevera confirmed the missing wiring.)

Fix

Add regex_pattern=args.regex_pattern to the slugify_params() dict. argparse already defaults it to None when the flag is absent, which matches slugify()'s own default, so existing behavior is unchanged.

Tests

• Added regex_pattern to TestCommandParams.DEFAULTS so test_defaults now covers it.
• test_regex_pattern: the flag flows through to params.
• test_regex_pattern_end_to_end: reproduces the exact example from --regex-pattern option ignored by CLI #175 and asserts ___this-is-a-test___.

Both new tests fail on the current main (KeyError / wrong output) and pass with the fix. Full suite: 84 passed.