Improve search configuration

These several commits mainly allow two new features :
* A big one : allow to define a score for each field, to define a score for every title (regardless of the corresponding model) and to define a score for each markup when the field contains HTML (or is wysiwyg).
* A smaller one : a callback can be defined to perform changes on a field value before spliting into words with scores.

I was not sure about scores that could be defined by default, I guess you can change this later.

As I did some changes, I dicovererd an unused feature, the one which disable search on specific words :) I therefore took account of them when refactoring "split_string()" method!

Hope everything's good for you.