Add option to center toaster text via props

[Nnanyielugo]

Hi, thanks for this great library.
However, it does not seem like there is the option to center the toaster message, rather than align to the left.
Something like pass in an option to center the text from props which will override the default text style by merely adding the style that centers the toaster message text.

If you like, I can open a PR that fixes this.