Solr configuration tunables

Add some tunables to the solr plugin, to play with the HTTP client when using the external Solr server.

@Primer42 @dallasmarlow @maddalab
SolrConfig.externalUrl.getOrElse(throw new CollinsSolrConfigException("We missed a validation somewhere")).toExternalForm

```
scala> import java.net.URL
import java.net.URL

scala> val url = new URL("http://www.google.com")
url: java.net.URL = http://www.google.com

scala> url.toExternalForm
res0: String = http://www.google.com
```