Enable `keyword search` in spoken word index

It would be great to have keyword search enabled for videos that are indexed through `video.index_spoken_words()`

There are two options we can provide this 👉 
- This can be a new index method and while calling `video.search` we can specify the index we want to use. 
- This can enable by default with `video.index_spoken_words()` and can be chosen when `video.search(type=`keyword`)`