v0.15.0

• Add context store
• KEMAL_ENV respects to Kemal.config.env and needs to be explicitly set.
• Kemal::InitHandler is introduced. Adds initial configuration, headers like X-Powered-By.
• Add send_file to helpers.
• Add mime types.
• Fix parsing JSON params when "charset" is present in "Content-Type" header.
• Use http-only cookie for session
• Inject STDOUT by default in CommonLogHandler