Implement start line number for source listing

[mojavelinux]

Implement a line number offset for source listing (specifically for Rouge, which supports this feature). To be consistent with ordered lists, we should use the attribute name start for this purpose.

[source,java,numbered,start=10]
----
public Long getId() {
    return id;
}
----

(Rouge uses the option name start_line and Pygments uses linenostart, so "start" seems to be the key term).

[mojavelinux]

There's a related issue for this in core. See asciidoctor/asciidoctor#1742.

[mojavelinux]

We can only support this for Rouge at the moment since Rouge is the only highlighter that currently supports line numbering.

[bwittman]

Pygments also supports line numbering, and I believe that it supports start line number through the linenostart option. Could this feature be extended to Pygments?

[mojavelinux]

The linenostart option from Pygments doesn't help us because of where Pygments puts line numbers. However, I found another way to implement it. I'm looking for feedback on the change. Please see #226.