Allow to override excerpt words

In thinking-sphinx 2 one could easily override the excerpt words like that:

```
excerpt_options => {:words => "mywords"}
```

Would be nice if that was supported in thinking-sphinx 3 too:

```
excerpts => {:words => "mywords"}
```

lib/thinking_sphinx/panes/excerpts_pane.rb

@@ -1,6 +1,7 @@
 class ThinkingSphinx::Panes::ExcerptsPane
   def initialize(context, object, raw)
     @context, @object = context, object
+    @excerpt_words = @context.search.options[:excerpts].delete(:words) if @context.search.options[:excerpts]
   end
 
   def excerpts