Summary downsize update

Introduces downsize v 0.0.8 and uses its contextual tags api to provide a round="true" (up)rounding feature to the content helper.

I've elected to implement this for the {{content}} helper first; the {{excerpt}} helper currently strips html before passing it to downsize. I want to think a bit about how smart we can be with that interaction, and I'd love to land this first.

###ce3e7b2
- downsize 0.0.5 -> 0.0.8
- downsize is now better at punctuation, update {{content}} and {{excerpt}} tests appropriately.
###16c38bf
- Passthrough to Downsize 'contextual tags' functionality.
- Allows for awaiting a markdown segment level closing tag before truncating content.
- Includes tests.

relates to https://github.com/TryGhost/Ghost/issues/4017

@ErisDS