Add to_dir option to render API

- add to_dir (destination directory) to render API
- add exception for exclusive option combinations
- add tests for the to_dir feature
normalize to_dir and to_dir + to_file

- normalize to_dir if running in safe mode
- normalize to_dir + to_file if running in safe mode
- make missing directories if specified
- update docs
- update tests
- fix return value for require_library
update comment
fail fast if cannot write file; simplify to_dir calc
update docs
Merge pull request #160 from mojavelinux/render-todir-option

Add to_dir option to render APIs