Add reading_time property to posts.

This is a work in progress, and I'm not sure if this is really a useful feature. 

The intent is to show a reading time along with each post, like [medium.com](https://medium.com/).  But, I'm not sure if this feature should be incorporated into the template or just leave it with the property being made available on the posts. 

Or one could just write the logic in the template. Not needing any of this code.