Resolves the basic part of wireviz#158.

Use this option value both in the graph and as font family in HTML.

The same color value is used for html.body.style.background-color
and gv.graph.bgcolor to make the diagram fit seamlessly in the HTML
output. "bgcolor" is chosen as option name to avoid the dash in the
CSS name "background-color".

Bug: Node attribute fillcolor is ignored unless style contains filled.
When cable nodes get an empty style (unless bundle), the node background
color become equal to the graph background color as it was transparent.

Fix: Make the style contain filled.

Bug: When the HTML label is narrow (e.g. connectors in tutorial01),
then the white node background is wider, and when the HTML label is
low (e.g. ferrules in tutorial05 and totorial06), then the white
node background is taller. Such errors are easily seen with a
non-white bgcolor.

Fix: Set node width, height, and margin to zero to let the actual
size of the node be entirely determined by the label.

It is default white unless set to a different color, or equal
bgcolor (as if transparent) if set to ~, null, Null, or NULL.

And a few other changes requested in the same review.

Co-authored-by: Daniel Rojas <[email protected]>