Margin and colour options for the view command

[wezm]

Also from #27 this PR adds the following options to the view command:

• --margin allows a margin to be added to the SVG. It can either be a single number, which will be used for all edges or 4 numbers, one for each edge.
• --fg-colour/--fg-color allows the fill colour of the glyphs to be specified as an rrggbbaa value
• --bg-colour/--bg-color allows the background of the whole SVG to be set as an rrggbbaa value

[wezm]

Some examples /cc @n8willis