Disable default centering (#47) and improve typings

[andrewsantarin]

So, learning that #47 is easily overcome with center={false} (I really, really thought that react-new-window is hardcoded to use some kind of centering), I wanted to improve the quality of life a bit by making center undefined at the start.

Also, I've added additional attribs to the IWindowFeatures interface so that it's clear what attributes go to the features prop. Referenced from react-popout-component, figured the interface could be useful since that library hasn't seen new merged commits in almost a year...

Feel free to roast my commit if it means writing even better code. :)