Query Explain

[k-capehart]

Follow up from here: #71

This feature, while useful, is still in beta as outlined here: https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/dome_query_explain.htm

With the risk of this functionality changing before leaving beta, I'm inclined to leave it in the feature branch query-explain until it has been fully released. After which I'll be happy to merge it in after verifying if any changes are needed.

@TuSKan Let me know your thoughts on that.