Add a --body-only option for webpage backend

[lthms]

Hi, and to begin with, thanks for alectryon. This looks like a very solid tool.

A little bit of context for backing up this feature request. On my website, I am currently using coqdoc to render Coq document as blogpost. I use the --body-only option of coqdoc, so that the rendered page is later postprocessed by soupault, inserted in a template, etc.

For quite some time now I’ve been willing to replace coqdoc with alectryon, notably to ditch coqdoc weird markup langage and use rst instead, and to get the nice proof features of alectryon of course. I tried this morning, but it looks like alectryon does not provide a feature similar to --body-only.

I had a look at hakyll-alectryon, but it looks like the approach there is a bit different, since the input file is a markdown one.

Any input is welcome. I can even try to implement the feature, if the conclusion is that this feature is welcome.