Template helpers

- [x] Relative urls: https://github.com/adamrenklint/asimov/issues/34
- [x] Deprecate markup plugins: https://github.com/adamrenklint/asimov/issues/18
- [ ] Implement Template helpers: https://github.com/adamrenklint/asimov/issues/9
- [ ] Allow use of (template) helpers in YAML substructures: https://github.com/adamrenklint/asimov/issues/31