Made inner loops have access to outer loop values

[ilyakooo0]

If my outer loop in a template produces a value that I want to use in my inner loop, I get a Missing field error.

This gives inner loops access to outer loop values and allows inner loops to override them.

[jaspervdj]

This looks good, thanks for the PR! Is it possible to update this with:

1. An additional comment in Hakyll.Web.Template.Context.listField, explaining how the contexts are combined
2. A test that reads a value from the outer context

[ilyakooo0]

Sure

[bergus]

I like it, had to do something quite similar in f49d972 (part of #620). Maybe this combineContexts function shouldn't be just local, but exposed in Context.hs?
Maybe the Context data type needs some general reworking though, if it is supposed to hold some information about an Item.

[ilyakooo0]

Yeah, I thought about exporting it. I thought it wasn't useful enough.

It sounds like a good idea. Now.

I'm a little overwhelmed right now, will finish the pull request in a couple days.

[bergus]

I guess for the API I would prefer a simple

bindItem :: Context a -> Item a -> forall b. Context b
-- or
bindContext :: Item a -> Context a -> forall b. Context b

(not sure which one is better?) over a three-parameter and non-descriptive combineContexts function.

You could then write

ListField c xs -> do
    sep <- maybe (return "") go s
    bs  <- mapM (applyTemplate' b $ c <> bindItem context x) xs
    return $ intercalate sep bs