Add titleColor option to notiflix confirm

[tomicapretto]

This PR is in response to the issue #1

This adds a the argument titleColor to use_notiflix_confirm().

Before:

shinypop::use_notiflix_confirm(
  okButtonBackground = "#367fa9"
)
#[...]
shinypop::nx_confirm(
  inputId = "confirm",
  title = "Confirm?",
  button_ok = "Confirm",
  button_cancel = "Cancel"
)

Now:

shinypop::use_notiflix_confirm(
  okButtonBackground = "#367fa9",
  titleColor = "#367fa9"
)
#[...]
shinypop::nx_confirm(
  inputId = "confirm",
  title = "Confirm?",
  button_ok = "Confirm",
  button_cancel = "Cancel"
)

[pvictor]

Thank you!