Custom Headers, HTTPS HMR, Error Handling

• @Filyus contributed a fix for HMR to work in HTTPS mode. #133
• @hanswilw contributed the new headers option to allow for use cases like CORS to work with custom headers. #132
• this.error for plugins now supports throwing errors inside an asynchronous function. #129
• Added support for e.frame for errors, so plugins throwing errors can show code snippets.