Fixed initialization failure when providing an array of elements.

When providing an array of elements to the initializer function, e.g. `captionator.captionify([document.getElementById("mainVideo")])`, it would call `processVideoElement` with an invalid `videoElement` because it used "spent" iterator value for indexing the `videoElements` array.

Since the rest of the code in the loop appears to work on an iterator value `videoElement`, it seems this is what should be used anyway.

This may also fix the following open issue: https://github.com/cgiffard/Captionator/issues/39