title: Add text shadow/outline to text labels in R plotly
body: <p>I'd like to see if I can add a black outline to white text in a <code>plotly</code> chart in <code>R</code>.</p>
  
  <p>I found the following link describing how it could be done in <code>plotly.js</code> but not sure it can be done in <code>R</code> as of yet. <a href="https://github.com/plotly/plotly.js/issues/1597" rel="nofollow noreferrer">https://github.com/plotly/plotly.js/issues/1597</a></p>
  
  <p>Here's a sample chart.</p>
  
  <pre><code>library(plotly)
  
  plot_ly(x = c('Product A', 'Product B', 'Product C'), 
          y = c(20, 14, 23), 
          type = 'bar', 
          marker = list(color = 'rgb(158,202,225)',
                             line = list(color = 'rgb(8,48,107)', width = 1.5))) %&gt;%
    add_annotations(text = c(20, 14, 23),
                    x = c('Product A', 'Product B', 'Product C'),
                    y = c(20, 14, 23)/2,
                    xref = "x",
                    yref = "y",
                    font = list(family = 'Arial',
                                size = 14,
                                color = 'rgb(245, 246, 249)'),
                    showarrow = FALSE)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/LhdHl.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/LhdHl.png" alt="Sample Plot"></a></p>