Allow callers to override :host, :port, and :scheme when generating urls
Add :absolute? key to url-for options, to force absolute URL
:absolute? key also forces scheme
Add tests for new linker capabilities
Flatten overrides in url-for to be top-level keys
Merge pull request #215 from pedestal/fix-linker-scheme

Allow callers to override :host, :port, and :scheme when generating urls