title: Flexdashboard: Pass reactive value to chart-title
body: <p>In the package flexdashboard the chart titles (headers for cells in the grid) are made via 3 hash marks (e.g., <code>### Chart title here</code>).  I want to pass a reactive value to this header.  Normally one could define a UI and push that (<a href="https://stackoverflow.com/a/48118297/1000343">https://stackoverflow.com/a/48118297/1000343</a>) but the hash marks are what tell the knitting that this is a chart-title.  I also thought about passing the reactive value using in-line code (e.g., <code>`r CODE HERE`</code>) as seen in the MWE below.  You can use inline text for chart-titles but not when it contains reactive values.  This results in the error:</p>
  
  <pre><code>Error in as.vector: cannot coerce type 'closure' to vector of type 'character'
  </code></pre>
  
  <p>In this case how could I pass the month in as a chart.title?</p>
  
  <h2>MWE (removing last line allows this to run)</h2>
  
  <pre><code>---
  title: "test"
  output: flexdashboard::flex_dashboard
  runtime: shiny
  ---
  
  ```{r}
  library(flexdashboard)
  library(shiny)
  ```
  
  Inputs {.sidebar}
  -------------------------------------
  
  ```{r}
  selectInput(
      "month", 
      label = "Pick a Month",
      choices = month.abb, 
      selected = month.abb[2]
  )
  
  getmonth &lt;- reactive({
      input$month
  })
  
  renderText({getmonth()})
  ```  
  
  Column  
  -------------------------------------
  
  ### `r sprintf('Box 1 (%s)', month.abb[1])`
  
  
  ### `r sprintf('Box 2 (%s)', renderText({getmonth()}))`
  </code></pre>