Support to display line number in a code block

[hawkchen]

When authors explain a code snippet, line number can help them precisely link their explanation to a specific code. Therefore, this can improve readers' understanding. Hope that a code block can have line number on it.

[dkotrada]

Same here I need it

[hackingbeauty]

+1

[starwavelin]

+1

[AaronO]

@hawkchen In the new release, all code blocks with 2 lines or more have line numbers (see screenshot)