Add option to search in specific collections

[nickgarlis]

This is an enchancement for search results. With this you can turn on/off search for specific collections with search: true/ search: false. In this example search is disabled for recipies. If you try searching for "chocolate" it will not come up with any results, however if you try searching for "oatmeal"
it will come up with the post in which search is enabled.

[mmistakes]

To avoid having to set search: true in every document (or as a front matter default), what if the default behavior is everything is included in the search index unless it has search: false?

Then you can do this instead:

{% unless doc.search == false %}
...
{% endunless %}

[nickgarlis]

Good idea, that would make more sense.