Contain the highlighted area when sample code is selected

It seems likely that a common thing to do on editorconfig.org would be to copy-and-paste some text from the sample (then edit it to suit your needs).

I thought it would be nice if the highlighting when text is selected didn't spill over the sides of the pre-formatted block.

Here's an example of what it looks like now:
![screen shot 2013-08-27 at 6 53 24 pm](https://f.cloud.github.com/assets/2081889/1039421/c9f86636-0f84-11e3-9270-94faea76a966.png)

And with the proposed change:
![screen shot 2013-08-27 at 6 53 50 pm](https://f.cloud.github.com/assets/2081889/1039423/d3cc6586-0f84-11e3-9ff1-1b0b4718e91c.png)

I tried to set the boundaries to emulate what it would look like if the selection were made in a text editor (i.e. just barely containing the characters on left side).