Introduced round="true" API to {{content}} helper

issue #4017
- downsize 0.0.5 -> 0.0.8
- downsize is now better at punctuation, update {{content}} and {{excerpt}} tests appropriately.
- Passthrough to Downsize 'contextual tags' functionality.
- Allows for awaiting a markdown segment level closing tag before truncating content.
- Includes tests.