Allow blank 'target' when specifying a custom router

[stuartthomson]

Is this a feature request?

Yes

Motivation

I have a proxy which needs to calculate the target at runtime, using infomation in the request, therefore in the options I specify a 'router'. However, it's not possible to run the proxy without also specifying a static 'target' option. This is forcing me to configure a random target which I never actually want to be resolved.

It seems like I'm not the only one who would find this useful, see here.

Thank you!