External code snippets: remove `data-include` attribute when rendered

[thomasheritage]

The instructions say:

data-include attribute on a pre element to the relative path of the snippet

Given that this is a relative path this will not be useful to most readers of the HTML rendering.

Therefore, it would make sense to remove this "editorial sugar" (per #146) from the HTML when rendered.

[SteveLLamb]

Not sure what the issue is here, on the documentation on how to do it or on the rendering? Is this even an issue at this point? @palemieux @thomasheritage ?

[SteveLLamb]

Oh, nevermind. I see what you mean.