Bulk "index" action and HEAD support

[tcrossland]

This PR includes support for the bulk "index" action (create or replace), while leaving "create" as the default.

I've also included support for HTTP HEAD requests, which Elasticsearch uses in various places (index exists, type exists, etc.)

[tcrossland]

Another small improvement to allow index settings to be specified as a map instead of a file path (to work around issues like #51 and #76)

[danielberkompas]

Approved when tests pass!

[tcrossland]

@danielberkompas thanks, I've changed the validation for settings to ensure that it's either a string (is_binary/1) or a map (is_map/1). I've removed the presence validation as it fails for empty maps (an empty map is now a valid value for settings)

[erikreedstrom]

It seems at least part of this functionality is covered under #68

We have a need to support delete actions as well in bulk, and would like to know which of the two PRs will be moving forward.

[erikreedstrom]

We've added some onto this in the expectation this is merged. 😄 We'll PR once this is in.

For reference, here's our changes compared to this branch: Bluetab/elasticsearch-elixir@feature/bulk-index-action...carsdotcom:decouple_bulk_action

[tcrossland]

@danielberkompas is there anything preventing this being merged? thanks

[michelboaventura]

Hey @danielberkompas thank you for the great project, it's been really useful for us. Unfortunately I'm hitting the issue this changes seem to address. Does this PR need anything else to be merged?

Thanks!

[danielberkompas]

@michelboaventura nothing else was needed, I just lost track of it!

[michelboaventura]

Thank you @danielberkompas!