Define custom levels

It would be nice to define custom log levels in a similar way to the full log4js to avoid TypeScript compile errors.

Using a custom log level (e.g.: `this.log.verbose('some msg')` causes a compiler error: Property 'verbose' does not exist on type 'Logger'.

Using `@ts-ignore` doesn't scale well and typing log4js as `any` is hacky and dangerous.
