Highlighting search results

Hi,

I'm submitting the pull request again, I made a mistake with the previous one. I added a highlighting feature, following your recommendations (issue #136). It adds two new configuration options : "Highlight_results" to enable the feature and "Highlighted_field" to specify which field will allow highlighting.

The wbr function was fixed to escape @KIBANA_XXX_XXX@ delimiters.

David