Feature: add line numbers to code blocks

[wscott]

I would like to have code blocks with line numbers.

I want to be able to add something to this:

apple
orange
banana

To make it look something like this:

 1 | apple
 2 | orange
 3 | banana

It would make it easier to talk about the source code snippet I pasted in a chat if I can refer to sections by line numbers. I would be OK if line numbers where just included by defualt (or with a per-orginization setting). Or numbers could be requested with an optional field after the highlightly option:
```c++,numbers

[timabbott]

This behavior in our markdown processor is controlled by the linenums flag. I'm not sure what the right model should be here for controlling whether or not to show line numbers.

[zulipbot]

Hello @zulip/server-markdown members, this issue was labeled with the "area: markdown" label, so you may want to check it out!

[certik]

I would love to see that too, came here to open an issue, but it's already open. :)

[ayala-io]

I would very much love to see this. HackMD (a popular cloud-based markdown editor) makes this optional by allowing the user to add an = to the source code snippet, for example:
```c++=
⮤This would add C++ syntax highlighting and line numbers

```c++=10
⮤This would add C++ syntax highlighting and line numbers with the starting line set to 10.

```=
⮤ No syntax highlighting, just line numbers.