organize safe modes in a module, align path normalization

- tuck the safe mode levels into the Asciidoctor::SafeMode module
- use Document#normalize_asset_path to build secure path for include file
- change docs on normalize_asset_path to be clear it handles files & directories
- align docdir attribute and base_dir property on Document
- add test for activated include macro
- test demonstrate that there are no regressions in security
Merge pull request #87 from mojavelinux/security-cleanup

Security tidying