Allow custom toast types

[agent0]

• I'm submitting a ...
  [ ] bug report
  [X] feature request
  [ ] question about the decisions made in the repository

• Do you want to request a feature or report a bug?
  Feature

• What is the current behavior?
  The creation of toasts is limited to pre-defined types

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via https://plnkr.co or similar.

• What is the expected behavior?
  I would like to create my own types of toasts

• What is the motivation / use case for changing the behavior?
  I want to have custom icons for some toasts

• Please tell us about your environment:

• Angular version: 5.2.0
• Browser: [all | Chrome XX | Firefox XX | IE XX | Safari XX | Mobile Chrome XX | Android X.X Web Browser | iOS XX Safari | iOS XX UIWebView | iOS XX WKWebView ]

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, gitter, etc)

The easiest way to implement custom types (as far as I understand the code) would be to make the 'add' method in toastyService public. The type that is passed as a parameter is appended directly to the toasts css class and I can easily define my own css for that.