Make loudRejection optional

[shesek]

As a followup for #93, this is a simpler change that makes loudRejection optional to allow users to define their own handling for uncaught rejections.

Without this, adding a user-defined handler that throws the exception in order to crash the process (i.e. process.on('unhandledRejection', err => { throw err })) could result in the log message appearing twice, depending on which handler gets registered first.

This is also the case with hard-rejection: enabling it after calling meow() results in two error messages, enabling it before meow() works as expected, which is somewhat of a confusing behavior.