Option to add very basic style customisation

[spwoodcock]

• The styling already looks pretty good!
• Perhaps we could add a basic CSS override to allow loading custom colour schemes and fonts?
• Allowing people to deploy stac-map alongside existing applications, to better align with required styling.
• Adding a logo / favicon would be a bonus too.

Options

1. Defined during build somewhere and bundled in.
2. Defined dynamically as part of config.json described here Predefined config at first load #136 (pointing to a CSS file to load - note this may cause a very small flash of unstyled content. Not sure if that's a dealbreaker).