Adds the ability to register a custom log formatter for HTTP logging, and addresses a CORS middleware bug.

CHANGELOG 7e0847f added ability to register custom log formatter (#131) 6257a58 Fix typo in cors.go (#127) 13a38d2 [bugfix] Handle CORS pre-flight request in middleware (#112) 2b8556b Revert "Add Vary header when allowedOrigins is * (#114)" (#122) 9327cfd Add Vary header when allowedOrigins is * (#114)c5874fa distinguish between explicit and implicit star (#118)