Don't index pages/posts containing specific categories/tags, possibility to limit shown results

[lorenzschmid]

Last PR for a while I hope ;) This one adds the following functionalities:

• Possibility to specify categories and/or tags which if a page/post contains them leads to its removal from the index.
• Possibility to limit amount of results which are shown upon search

The first functionality is defined in the _config.yml file with the excludes property which new is a hash and contains three keys:

• files: current behavior with regex
• categories: newly added
• tags: newly added.

Still, using the excludes property as an array (as done until now) it will automatically be "transferred" to the new, hash-based configuration and therefore not break any older system.