rewrote slugify func with radon

[IvanChumakov]

Made some changes in slugify function using the Radon utility (reduce the cyclomatic complexity of the function)

[marksteward]

This is probably worth doing, but it could do with some tidy-up.

In particular:

• the docstring has vanished
• _ensure_unicode is redundant since python3
• _truncate_text takes default_separator instead of referring to the global DEFAULT_SEPARATOR
• _adjust_separator takes both default_separator and separator
• _apply_replacements, _adjust_case, _process_stopwords and _adjust_separator all do an early return if not enabled on entry. This would be clearer outside the functions, and shows that _adjust_case really just obscures a call to .lower().

I also think _safe_sub should be wrapping repl, not re.sub, but this would be a change in behaviour.