Added a 'show-forever' mode and added a delay for manually close()

[itinance]

This PR adds the feature to show a Toast forever until a separate call to "close"-method.
If the modal is shown "forever", a slightly delay of 250 ms was added to the close()-method to avoid flickering effekt if the close()-call comes very fast. This special delay can be changed as a parameter to the close method and also as a prop to the toast-component

I've provided also a README-section for "forever"-mode with example code for further information.

This PR is backward compatible and will not break anything.

[crazycodeboy]

Thanks,you can use these features in the 1.0.6 version。