Add support for navigation between articles

[htdvisser]

Can use:
.Prev Pointer to the previous content (based on pub date).
.Next Pointer to the following content (based on pub date).
.PrevInSection Pointer to the previous content within the same section (based on pub date)
.NextInSection Pointer to the following content within the same section (based on pub date)

See http://gohugo.io/templates/variables/