Release 0.6.0

Features:
- Add `wordpress::wp_additional_config` parameter for custom template
  fragments.
- Add `wordpress::wp_table_prefix` to customize the table prefix in
  mysql.

Bugfixes:
- Fix idempotency for `mysql_grant` privileges.
Merge pull request #25 from hunner/release_0.6.0

Release 0.6.0