Zoom shiny leaflet world map to full width

Does anyone know how to set default render zoom of a Shiny Leaflet map so a full world map zooms to fit the window width (i.e. -180° to 180° properly fits the window)? I can only set zoom to integers, with 2 being too small and 3 too big. Reproducible example:

require(shiny)
require(leaflet)
require(magrittr)

d = data.frame(country = c('China', 'Brazil', 'Canada'), lon = c(105, -52, -95), lat = c(35, -10, 60))

server <- function(input, output, session) {

  output$mymap <- renderLeaflet({
    leaflet(d) %>% 
      addCircleMarkers(layerId = ~country, lng = ~lon, lat = ~lat, label = ~ country, radius=30) %>% 
      addProviderTiles(providers$Stamen.TonerLite, options = providerTileOptions(noWrap = TRUE,  minZoom=1, maxZoom=18)
      )
  })
}

ui <- fluidPage( 
  tags$head(tags$style(HTML("#mymap { position: fixed; left: 0; top: 0; width: 100vw; height: 100vh !important; }"))
  ),
  leafletOutput("mymap") 
)

shinyApp(ui, server)