add config option for node hostname

[willnorris]

this overrides the name used to refer to the node in the caddy config, and is mostly useful because it can include environment variables.

Closes #18

[irbekrm]

Nice, I tested it with a simple Caddyfile

{
	tailscale {
		foo {
			hostname "{$FOO}"
		}
		bar {
			hostname "{$BAR}"
		}
	}
}

:8081 {
	bind tailscale/foo
	respond "hello foo"
}
:8082 {
	bind tailscale/bar
	respond "hello bar"
}

and got the expected two Tailscale nodes with hostnames set to the respective env var values.

Suggestion: should we add hostname to the nested node config in the top level example Caddyfile for documentation?
Suggestion2: should we have a test case for this?

[willnorris]

added tests, as well as for the previous control_url change, which I missed. I'll document all of these new field together in an upcoming change.