Ability to apply filter by metadata without affecting scoring

[fabito]

First if all, thanks for the library, very useful and easy to setup.

We are working on a multitenant application and we are planning to use the metadata property to add the tenant_id.
When searching for matches we need to do it in the context of a specific tenant.

The idea of this PR is to add a new (optional) filter parameter to the search_image method, so that drivers could apply it before actually trying to find matches.

A few questions:

1. Does it make sense ? Are you interested in such a feature ?
2. I'd like to write some tests for the ES implementation. Can anyone instruct me how do I run the integration tests ?
3. I have no idea how to add this feature to the Mongo driver. Is it a problem ?

[rhsimplex]

First of all, thank you for @fabito not only for the PR but also the test coverage 😍!!

I'm on vacation for the next week and a half, but I will try to have a look at it soon. So please be patient if you don't hear back from me for some days!

To answer your questions:

1. Yes, this is an excellent idea for a feature, and I think others will use it.
2. Don't worry about mongodb, I don't have test coverage for it and I don't think anyone is using it.

[rhsimplex]

Thanks again @fabito