FEATURE: Add multi match search

[unclecheese]

This allows searching across multiple fields, and boosting them, if required.

$elastic = Injector::inst()->get('SilverStripe\Elastica\ElasticaService');
$results = $elastic->search($query, array('Title^3', 'Content'));

I'm 90% sure this is not going to be your preferred solution, but I figured best to get the dialogue started.

Another approach would be to add a new method, searchMulti instead of the polymorphic approach.

[nyeholt]

Andrew's not maintaining the module directly any more, so I'm happy to take in what works for you :) If/when we look at re-working things, it'd probably use a query-builder type of object anyway, so until then I'm happy for things to just be 'whatever works'.