Allow customizing top-bar colors

Provides a way to allow implementors to specify custom
colors for the top bar branding area of a site by changing
the region.json config file.  Simpler than adding a pre-compile
LESS/SASS step and cleaner and more accessable than attaching
`!important` overrides to implementation-specific css files.
Merge pull request #164 from CoastalResilienceNetwork/topic/custom-styles

Allow customizing top-bar colors