Edits to make novice R lesson 01 more similar to novice Python lesson 01.

I have made edits so that the first R lesson more closely follows the Python lesson. It covers all the same topics save two:

  + Loading external packages - omitted because none of the operations in the lesson require loading an external package.
  + How to display three plots side-by-side - omitted because this is more complicated in R. This would require discussing how to set and reset `par` options, which is not a beginner topic.

Here's how you can quickly compare the two rendered files:

```
# clone bc repo
git clone git@github.com:swcarpentry/bc.git
# create and switch to new branch
git checkout -b review-pr
# add remote
git remote add jdb git@github.com:jdblischak/bc.git
# pull changes from my branch
git pull jdb 01-starting-with-data
# build site locally
make site
```
Then compare the rendered files  `_site/novice/r/01-starting-with-data.html` and `_site/novice/python/01-numpy.html`.
Cool, though an extra newline after the `<code>` and before `x` would probably fix the Github rendering (unless that's for consistency sake too).  

Hmm, can you link me to an example python though? I missed the part about switching into using `<pre>` html tags in the output.  It seems non-ideal to me to have this chimeric markdown-html mix for code blocks, (even though I suppose the .md is just an intermediate form at this point that no one cares about.)  Using `~~~r` you have a syntax that every just about every markdown parser knows about (your Jekyll flavor: kramdown, pandoc, and Github flavor, anyhow), which should result in nice pretty syntax highlighting on Jekyll presuming you've set it up.  Using explicit html "pre" blocks you have a far more ambiguous format that doesn't seem indicate the language used.