add `removeToast()` and `clearToast()` functions

[merlinoa]

These functions should call the toastr.remove() and toastr.clear() methods in the toastr JavaScript library. They are useful for removing toasts messages after something happens. e.g. if you set the "timeOut' and "extendedTimeOut" options to 0, the toast will not be removed until either the user clicks the toast or it is removed with one of these methods.

[phoward38]

• Add keep_visible argument to showToast, set to FALSE as default
  • When keep_visible=TRUE, overwrites timeOut & extendedTimeOut values in list for .options arg & sets to 0

[phoward38]

Completed & created PR #44