Add UpdateMany for BulkOperation

[dimmasik]

NEST/Elasticsearch.Net version: 5.0

Elasticsearch version: 5.0

Describe the feature: Add UpdateMany to NEST like IndexMany, CreateMany and DeleteMany

[russcam]

@dimmasik fancy submitting a PR for this?

[dimmasik]

yes, i can

[russcam]

Awesome @dimmasik! Take a look at the contributing guide. Let me know if you need any guidance in getting the solution up and running or have any problems with it. For Elasticsearch 5.0, if you make the PR against the 5.x branch, I can pull it into the others