Add line numbers to code blocks

An option to format code blocks with line numbers (or make code blocks always have line numbers).

When someone posts a code block, currently the only way to tell them what needs to be changed is by repeating the same line that needs changing. If there was an option to add line numbers to code blocks, it would be as simple as just stating the line number (or range of lines) that need attention.

Since this is something that isn't really required all the time, make it optional, for example, by adding a * (or any other applicable symbol) at the end of the starting triple-backticks. Example

```cs*
// C# code block
```

or when no language is specified

```*
Some text formatted like a code block without any syntax highlighting
```

Make line numbers always start from 1 for easy referencing.