React-toolbox: Snackbar icon

[MarcoWorms]

According to Material Design guidelines snackbars can't have icons https://material.google.com/components/snackbars-toasts.html#snackbars-toasts-usage

Snackbars should contain a single line of text directly related to the operation performed. They may contain a text action. They cannot contain icons.

The main example of snackbar usage has an icon http://react-toolbox.com/#/components/snackbar

I could PR this but maybe you guys don't wanna change that :)

[anri-asaturov]

Wow, aren't you a purist.

[rubenmoya]

Well, the spec is the spec, I'll open a PR soon :)

[MarcoWorms]

Thanks man!