Don't match fenced code block that has four backticks

[mojavelinux]

Do to a logic error in the parsing code, 4 backticks are being recognized as the start of a fenced code block, but only 3 are searched for the trailing delimiter. This causes the remainder of the document to break.

Create a negative test case for the following snippet.

````ruby
puts "Hi"
````

Make sure that doesn't match, or break the document.

[mojavelinux]

While we're at it, honor the the line number setting after the first fence:

```xml,numbered
<root/>
````

Any value in the second position after the comma should be interpreted as enabling line numbers. This covers all three positional attributes for the source block (the first, the block style source, being implied).