Add HTML content to app launch overlay

Add HTML content on top of the semi-transparent full screen overlay shown at app launch.

app_overlay(text, logo = NULL, loading.logo = NULL,
  text.loading = "<h1>Loading...</h1>")

Arguments

text	character, typically a string of html.
logo	character, path to image.
loading.logo	character, path to image.
text.loading	character, text to accompany `loading.logo` or alone.

a shiny::HTML string.

#not run