Expose remote address as environment variable

[davekok]

Is your feature request related to a problem? Please describe.
For simple configure/maintance bash scripts running over a term.js it would be nice to have some knowledge of the user agent. So this can be used in configuring the environment for the user. Like setting up a dev environment where xdebug needs to know the remote address of where the client (IDE) is listening.

Describe the solution you'd like
Expose HTTP information headers as environment variables to the command ttyd execute. By preference through an option.

Describe alternatives you've considered
Autodiscovering the client through varies means, asking the user for their IP address. None work in all situations, like when running in a container. IDE try to pass the right information but without indepth knowledge of the container environment can't always get the right one.