New features.

• Enables to accept 'application/json' request content type. #35
• Enables to load html files from S3. #41
• Enables to set timeout for reverse proxy request. #40
• Add authentication methods. #43

Refactoring and internal changes.

• Switch to Echo (Goodby Rocket). #33
• Migrate to aws-sdk-go-v2. #34
• Split go's package from main to mirageecs and mirageecs_test.
• Add E2E test for webapi.

Breaking changes.

• Response format of /api/purge.
  • from {"status":"ok"} to {"result":"accepted"}