Possibility of configuring index name at run time?

[Brian Glusman]

Not sure how common a use case this is, but this library looks great for my needs other than one small problem, our model doesn't have one index, but per-company indexes... so to use the Document module, we can't define @es_index at the module level, but invocation time of methods... I can work on a PR to add this capability if you're open to it, or if you have time and know how you'd like it done that would be awesome! (could do same for @es_type but that seems less likely to be invocation independent...)