Add reading_time property to posts.
Add reading_time property to posts.

The property returns a rough estimate of the reading time for the blog
post in minutes.  This code is inspired by
https://gist.github.com/zachleat/5792681.

Note: This commit does not make any changes to the templates

andredias/nikola,damianavila/nikola,gwax/nikola,yamila-moreno/nikola,wcmckee/nikola,schettino72/nikola,lucacerone/nikola,TyberiusPrime/nikola,techdragon/nikola,pluser/nikola,gwax/nikola,lucacerone/nikola,techdragon/nikola,JohnTroony/nikola,s2hc-johan/nikola,yamila-moreno/nikola,pluser/nikola,wcmckee/nikola,xuhdev/nikola,xuhdev/nikola,gwax/nikola,immanetize/nikola,xuhdev/nikola,atiro/nikola,okin/nikola,schettino72/nikola,s2hc-johan/nikola,getnikola/nikola,getnikola/nikola,JohnTroony/nikola,andredias/nikola,x1101/nikola,pluser/nikola,okin/nikola,yamila-moreno/nikola,okin/nikola,getnikola/nikola,jjconti/nikola,jjconti/nikola,knowsuchagency/nikola,immanetize/nikola,berezovskyi/nikola,masayuko/nikola,berezovskyi/nikola,lucacerone/nikola,Proteus-tech/nikola,schettino72/nikola,TyberiusPrime/nikola,damianavila/nikola,berezovskyi/nikola,Proteus-tech/nikola,masayuko/nikola,x1101/nikola,atiro/nikola,x1101/nikola,s2hc-johan/nikola,damianavila/nikola,xuhdev/nikola,wcmckee/nikola,knowsuchagency/nikola,JohnTroony/nikola,andredias/nikola,immanetize/nikola,TyberiusPrime/nikola,atiro/nikola,Proteus-tech/nikola,techdragon/nikola,jjconti/nikola,knowsuchagency/nikola,Proteus-tech/nikola,getnikola/nikola,okin/nikola,masayuko/nikola