Allow custom agent

[fabiob]

Sometimes we need to customize the agent used to communicate with Solr. In my specific use case, I need to enable HTTP keep-alive, but I can think of other uses.

The custom agent could be specified when creating the client:

var client = new SolrNode({
    host: '127.0.0.1',
    port: '8983',
    core: 'test',
    protocol: 'http',
    agent: new https.Agent({ keepAlive: true })
});

[ted-lee]

I need this feature, too