Add Labeled tunnels

[jtszalay]

resolves https://github.com/mohammed90/caddy-ngrok-listener/issues/8

This PR adds the following:

• Labeled tunnel type.

• lables can be defined in a block:

  label {
  edge edge_34242
  test labeltest
  }

  or inline:

  label edge edge_34242

  or a combination of both.

• Basic sanity tests for labeled tunnels.