Add theming support

[fbanning]

Yeah, sorry I wasn't clear. It may be convenient to allow the user to pass a dictionary to osmplot! (or to propagate keyword arguments) so that the user can decide the color of the street types instead of the individual edges.
Originally posted by @Datseris in #8 (comment)

We have a pretty basic default theme in src/defaults. In default_consts.jl we for example define some dictionaries for colours, widths, and styles of edges depending on their way type ("motorway", "pedestrian", ...). These attributes are then used in defaults.jl for construction of the related edge attributes.

It would be very nice to implement some kind of basic theming support for users to change the colourscheme, widths, etc.
There might be two possible ways to do this:

• Stick to current approach. Users can provide dictionaries of which values to change with fallbacks to default theme. This is easy to do with the current way to theme the plot and can be done as a first step.
• Completely switch to using a Makie theme so that users can use Makie.set_theme! or Makie.with_theme. This is a more fundamental change and will require some research into Makie themes. I'm not sure if it provides all the detailed layers of theming that we require.

[fbanning]

Implementing MapCSS support would make me happy.