Fix the Handler argument types missing request.params

The inferred types for Handlers were missing the ctx.request.params property, resulting in an error when attempting to access them.

Also fixes the default value for KoaCompatibleMiddleware's generic type CustomT (custom context) being wrong. The previous type was equivalent anyway, but it's still a misleading hint for developers.

Fixes #26

Coverage remained the same at 88.889% when pulling 3e306ed7b62fe1a4a49491a050bfb7886f969741 on opl-:fix/context-types into 7d6c930b14d8915f7567d435f29b69237aa17a9d on therne:master.