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"}
```
Option to override excerpters words belongs to excerpter
Option to override excerpters words
Merge pull request #551 from groe/patch-1

Allow to override excerpt words