feat(get): add _get function

[eunjae-lee]

This PR adds a method _get for internal usage.

Currently we cannot get the instance variables because the main interface is a function (aa()).
The initial reason for this functionality is because I need access to _hasCredentials.

The reason why I didn't make this function return a value, but make it accept a callback is,
it won't work in umd build.

!function(e,a,t,n,s,i,c){e.AlgoliaAnalyticsObject=s,e[s]=e[s]||function(){
(e[s].queue=e[s].queue||[]).push(arguments)},i=a.createElement(t),c=a.getElementsByTagName(t)[0],
i.async=1,i.src=n,c.parentNode.insertBefore(i,c)
}(window,document,"script",ALGOLIA_INSIGHTS_SRC,"aa");

According to the snippet,

window.aa is basically a function adding commands to the queue.

function () {
  (e[s].queue = e[s].queue || []).push(arguments);
}

In order to have the same signature across the browser and node env, I've decided to pass the callback function.

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[francoischalifour]

What about having actual public APIs? (getAppId, getApiKey, etc.). Having a generic method that returns private data is not very safe because we're not in control of what users can retrieve.