Added possibility to version indexes.

[Boorinio]

Hi everyone, This is a simple pr that adds the indexableAs method in the searchable trait.

Why?

• Rolling updates: It ensures zero downtime by indexing data into a new index while searching continues on the old one. This is something I feel is missing from scout, especially for high traffic websites that want to use it and it is quite easy to achieve this way.
• Backwards compatible: users that don't care for this feature wont be affected since by default the indexableAs and searchableAs have the same return.

Usage scenario:

1. Users Model is searchable
2. You define the indexableAs with return 'users_v2'
3. You reimport everything in users_v2, meanwhile your website can still perform searches on the old index
4. You deploy a new version of your code with the searchableAs returning 'users_v2'
5. profit!

[Boorinio]

Thank you, much appreciated!! @taylorotwell