💡 Custom headers.

**Describe the feature you'd like**
Allow user to add custom header(s) when `cloudflared` connect to the local service(s).

**Describe alternatives you've considered**
Yes, I'm using nginx to do that for now, and actually adding custom header(s) is the only thing that nginx do, I really wanna make `cloudflared` connect to the local service(s) directly.

**Additional context**
If `cloudflared` already support to do that, could you please tell me how should I do or where I can find the docs?