Improve the default host value

[stof]

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• add support for passing the ES host in a query string parameter, allowing for a better experience for standalone deployments (i.e. when running this site by pointing a webserver to the _site folder rather than using ES to run this webserver for a site plugin)
• use the current location only when the site runs as an ES site plugin

[stof]

Note that I'm not sure the code itself would be compatible with ES 5.x yet (there might be BC breaks affecting it). But the standalone deployment is a way to try it (even though I'm not yet using ES 5.x).

Note that I took the idea of a standalone deployment from https://github.com/royrusso/elasticsearch-HQ.

[stof]

@polyfractal do you have any interest into merging this ?