Integration with  downsize ContextualTags API.

This commit is an initial attempt to integrate with the downsize contextual tags API ([commit](https://github.com/cgiffard/Downsize/commit/00f9fd88f6a0d0a7261f4d037dedd119e8d899ef)).

This implementation would allow you to pass context="true" to {{content}}, and not get your 'core' markdown generated elements clipped. i.e. the elements p, ol, ul, pre, and blockquote would all not be clipped. Instead, whatever element the character limit or word limit was hit inside of would be finished before the html is clipped.

I've tried to adhere to code and API conventions, please let me know if there's anything you'd like me to change! (This is my first ghost PR, so I'd love feedback :))

Relates to:
* https://github.com/TryGhost/Ghost/issues/2206
* https://github.com/TryGhost/Ghost/issues/2166

Best regards,

-adam

##commit message:
Introduces the context="true" API to {{content}}.
Currently defaults to false.