viewer: add colour theme support

[abey79]

In a first step towards better colour support and to address abey79/vsketch#51, colour "themes" should be added to the viewer:

• a "theme" would have a name and contain a background colour + colours of the N first layers (layers > N have the colours of their modulo N)
• a few themes would be included by default ("standard", "cmyk", "black")
• additional themes could be defined in ~/.vpype.toml
• theme would be selected from the layer "button menu" in vpype viewer
• selected theme would be remembered across vpype instantiation

What this issue explicitly doesn't cover:

• adding colour metadata to vp.Document
• reading colour from input SVG (#35)
• setting colours of output SVG (#145)

[abey79]

This is better covered by #359