Snippet enhancement: follow recommended practices for delimited blocks

[PrimaryFeather]

First of all, thanks for this fantastic extension for Sublime text! That's simply the best way to write AsciiDoc. 😄

I've got a suggestion regarding the snippets: currently, when you create a listing or quote block, a huge number of delimiters is created. E.g.

[source, ruby]
-------------------------------------------------------------------------------

-------------------------------------------------------------------------------

However, the official recommended practices say that you should use no more than 4 such characters.

[source, ruby]
----

----

Since this is the official AsciiDoctor package for Sublime, I think it would make sense to follow these guidelines. If you want, I can send a pull request with the necessary changes.

Thanks a lot in advance for considering this small change!