Minileven: Add a setting to display featured images.

Search results will not display featured images regardless of setting value.
Minileven: Add filter to allow filtering of where Featured Images are displayed.

Merges ideas from and resolves #619.
Minileven: Disable Featured Images on single view and pages by default.
Merge pull request #711 from Automattic/kwight-minileven-featured-images

Minileven: Add a setting to display featured images.