Add support for thumbnail in the post preview

The thumbnail is inserted in the font matter via the param `thumbnail` which should contain the image source.

The style of the thumbnail is controlled by two css variables:

- `thumbnail-size`: default is 150px
- `thumbnail-position`: default is `1` which places it to the right, can be set to `-1` to place it to the left

Also updates some of the content post to show the effect.