Add support for linenostart

[bep]

I know I'm pushing it in the feature request department, but I assume this should be easy to add.

In Pygments, the flag is called linenostart -- which when set makes the line number start at a different value than the default 1. I assume you can add it as an argument to WithLineNumbers.

[alecthomas]

I did see that in Pygments, but couldn't really understand what it is for. What is the use case for it?

[bep]

What is the use case for it?

If I want to document some existing big Java class (or whatever) and copy and paste, say, lines 46-200, into my documentation. In that case it would make sense to have it start at counting from 46 so it matches the original file.