Option for settings listings code block environment name

[aviramsegal]

Added --listings-env option to allow settings the name of the code block environment when using listings.

This allow using a template with \lstnewenvironment for settings a custom environment.

I tested manually and with test-pandoc and everything passed

[jamtur01]

Might also want to consider an example of how to use it in the template perhaps? Or docs.

[aviramsegal]

Good point @jamtur01 but I am not sure about where to do that, if @jgm decide to accept this pull request I'll consult with him about the best way (maybe as an example or in the User guide on the website)