This reverts the author front matter parameter back to a string in order to fix the 12 theme demos that broke with #33
The demos that were supposed to be fixed in that PR were whiteListed with https://github.com/gohugoio/hugoThemes/pull/586
This PR adds a `type = "posts"` for themes that need it.
Most notably [Hyde Hyde](https://themes.gohugo.io/theme/hyde-hyde/) that currently has its index page empty will have it populated with posts once again.
If more themes need different content types I will see whether it is possible to turn `type` into an array to add more of these.
