title: Get a second header with the units of columns
body: <p>Sometimes in academic texts one wants to present a Table in which every column has units. It is usual that the units are specified below the column names, like this</p>
  
  <pre><code>|Object       |Volume  |   area  | Price    | 
  |             |$cm^3$  |$cm^2$   |   euros  |
  |:------------|:-------|--------:|---------:|
  |A            |3       |    43.36|    567.40|
  |B            |15      |    43.47|   1000.80|
  |C            |1       |    42.18|      8.81|
  |D            |7       |    37.92|      4.72|
  </code></pre>
  
  <p>How could I achieve this for my <code>bookdown</code> documents?</p>
  
  <p>Thank you in advance.</p>