cmd/preguide: support decorators

[myitcv]

Placeholder for decorator support.

We currently vary the output of a step in the declaration of a step itself:

preguide/cmd/preguide/testdata/renderers.txt

Lines 88 to 98 in 78bc677

	Steps: step1: preguide.#Upload & {
	Target: "/home/gopher/somewhere.md"
	Renderer: preguide.#RenderLineRanges & {
	Lines: [[2,2]]
	}
	Source: """
	This is some markdown `with code`
	Another line
	A third line
	"""
	}

This is wrong. Applying ellipsis, or rendering a specific range of lines, is a purely presentational thing.

We should therefore be doing this in "decorators" that modify the result of the step function. Something like:

{{ step "step0" | lineRange [[2,2]] }}

• Add support for decorators
• Convert all existing renderers to decorators