Different pipelines for indexing and searching.

The possibility of using different pipelines for indexing and searching would be required for the implementation of some smart token filters/tokenizers - see #80 for an example). 
This commit demonstrates the idea. 
Unfortunately, this would imply a change in the basic lunr API, but as it has not reached version 1.0 yet, you could consider to include this feature in your library. (In this case, we have to change the API docs too)