Streamline listing inclusion

[ibnesayeed]

As per the README:

[TODO: We need to add a folder for external code samples, streamline the mechanism to add listings, include a list of listings after ToC in the class, and add some helper macros.]

[machawk1]

I agree but should we really call this "listings"? I ask in respect to the case of verbatim usage in lieu of listings where a text snippet is included that is not code, console readout, etc. Maybe "snippets" but even that is vague.

[ibnesayeed]

We can certainly think of a better naming here. :)