support of html_head and html_tail config keys

Eventually did the following:
- Forked your repository and replayed my changeset
- Derived t/05-head_n_tail.t from t/01-usage.t for config is read during initial use only
- Both files might be factorized for they are really close indeed
- Bumped version to v0.12_01 for traceability's sake

HTH.