add show'ed version of the Options record to diagram source hash

This addresses part 1 of #3.  Now diagrams will be rebuilt even if
only their size changes (or any other backend-specific options).  This
does require adding a Show constraint for the Options record but for
most backends this shouldn't be a big deal (I've already added such
instances for the svg, cairo, and postscript backends: see
diagrams/diagrams-svg@95cbdc62da549a87ce3d699fd9299902fee16fa2,
diagrams/diagrams-cairo@e382d4b15c3e0c9bc56f102709aebe941e5a772c,
diagrams/diagrams-postscript@e862b3bcfe4d04c20648a1d4d9d60aa02fe9d9db).
bump version to 0.4

Note the major version bump is required by the PVP since the type of
buildDiagram has changed.
minor whitespace cleanup [ci skip]