Set text selection color to match Monokai theme

I got the highlight color from `/Users/joe/Library/Application Support/Sublime Text 2/Packages/Color Scheme - Default/Monokai.tmTheme`

Specifically lines 20 and 21:

```xml
<key>lineHighlight</key>
<string>#3E3D32</string>
```

I then converted the hex values to rgba and set the opacity to 0.5, which looked pretty close.