closes#9200
- Registered new server helper `{{reading_time}}`.
- Added new global util `word-count` based on the util in Ghost admin, which returns the number of words in an HTML string.
- Based on the word count of the post html, the helper calculated the estimated reading time:
- 275 words per minute
- additional 12 seconds when post has feature image
- Renders a string like 'x min red', unless reading time is less than a minute. In this case, the rendered string is '< 1 min read'.
