Add a configuration to allow forward to a different local host

[antoniomika]

Adds the ability to provide a different host to forward connections to. Closes #14.

Can test with the following:

# Start a tunnel proxying a port to httpbin.org:80
cargo run local --to bore.pub -l httpbin.org 80

# Access a url that httpbin.org:80 exposes
curl -vvv http://bore.pub:<port>/anything

[ekzhang]

Thanks, this looks great! Only one minor thing (&str usage) and please if you could run cargo run help local and cargo run help server and update the usage section fo README.md, that would be very helpful.

[antoniomika]

Updated, thanks for the review!

[ekzhang]

Thank you! :)