codeblock plugin: start the line numbering with any value (default: 1)

Starting the line numbering with a value different than 1 is useful when the codeblock contains just a fragment of a larger file.
See the screenshot:
![screen shot 2013-08-05 at 12 46 00](https://f.cloud.github.com/assets/1086598/909142/f23653a2-fdb3-11e2-803e-33fb7dc769c8.png)