Support an arbitrary line number start value for specific code blocks

[phrohdoh]

Something like the following, if possible:

```yaml,line_num_start=5
5: foo: bar
6: ...

Where the line numbers start at the specified value, 5, instead of 1.

---

Combined with #5 we could get something like:

```yaml,line_nums,line_num_start=5
5: foo: bar

Which would force line numbers to be rendered even though it is a one-line and start the line numbers at the specified value.

This is useful when you are dissecting a code-block line-by-line, for example, and want to make the context clearly known.