Define custom levels

[kevintechie]

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.