Provide defaultCurrent

[kostia1st]

To make this component a bit more flexible and SSR friendly, I suggest adding a defaultCurrent prop.

Currently, state.current initializes with 0, always. That means, the only way to influence it, is to call setState, and setState could be called just on a mounted component (in client's browser), after the initial render. And the initial render renders the very first image even if it's not the wanted behavior.

In my use case, I need to have a very specific image as the default one, not the one that comes first in the image collection.

What do you think?

[andrepolischuk]

cool, it will be added in #21