Allow explicit connection passing

[jasongilman]

It looks like elastisch only allows use of a global connection to elastic. This breaks cases where an application would want to use multiple connections to elastic. The API functions should take an optional connection in cases where a global connection is not appropriate.

[michaelklishin]

One problem with this is that we already have 2 clients and there are many features in ES. So adding alternative API namespaces will quadruple the API, in some sense (there will be 4 variants of the same function).

So I'm inclined to make client a mandatory argument for 2.0.

[michaelklishin]

Working on this today and tomorrow. Stay tuned.

[michaelklishin]

It turns out to be much more time consuming (mostly updating the tests) but I'm making progress.

[michaelklishin]

REST client is converted, native is in progress.

[michaelklishin]

In master. Now we need to update all the code examples in the docs 🎌.

[michaelklishin]

The docs are updated.

[jasongilman]

That's awesome. I didn't expect such a fundamental change to be made so quickly or without pushback. This is going to make the library a better fit for many different use cases.