Have ggplotly() replace default fonts with thematic's font option

[cpsievert]

Closes #1803, here's an example:

library(shiny)
library(plotly)
library(thematic)

thematic_shiny(font = "auto")

ui <- fluidPage(
  titlePanel("Hello auto theming with ggplotly()"),
  tags$head(
    tags$link(href="https://fonts.googleapis.com/css?family=Pacifico", rel="stylesheet"),
    tags$style(HTML("body{background-color:#444444; color:#e4e4e4; font-family: Pacifico}")),
    tags$style(HTML("a{color:#e39777}"))
  ),
  plotlyOutput("p")
)

server <- function(input, output, session) {
  output$p <- renderPlotly({
    p <- ggplot(faithfuld, aes(waiting, eruptions, z = density)) + 
      geom_raster(aes(fill = density)) + 
      geom_contour()
    ggplotly(p)
  })
}

shinyApp(ui, server)