Responsiveness for Toaster Messages

[RangarajKaushikSundar]

@tomchentw I have added a few media queries to the toastr.min.css and want to share it here. I will not be able to add it to this project since that was added as an external file from cdnjs. Please advise.

[tomchentw]

We could have our own file in this repo. Say: src/toastr-responsive.css, and our user can import it via:

@import url(~toastr/toastr.min.css);
@import url(~react-toastr/lib/toastr-responsive.css);