Add whitespace between RStudio markdown chunks

A little white space between R Markdown sections gives the reader a little pause in the story before changing the subject otherwise all the cells get smooshed together by default and it ends up looking like one long run-on sentence.

Using CSS is a great way to create whitespace consistently and easily.

Example

```{css echo=FALSE}
/* Define a margin before heading elements */
h1, h2, h3, h4  {
  margin-top: 12px;
}

/* Define a margin after every first p elements */
p:first-of-type {
  margin-bottom: 6px;
}
```