feat: Environment variables support

[domwst]

First thing first, thank you for an awesome tool, it simplified my life a lot.

On macos the SSH_AUTH_SOCK variable is changed on between startups, it would've been very handy if one could specify SSH_AUTH_SOCK as an environment variable in config like this:

agent_sock_paths = [
	"${SSH_AUTH_SOCK}",
	# ...
]

There is also a bit that is responsible for config generation, not sure what's desired behavior there, it should either

• Add current value of SSH_AUTH_SOCK into the config (as it is now)
• Add the placeholder and hope that this variable is set on daemon startup

Not sure if the second option is viable on every system though

[overhacked]

This is a great enhancement, thanks! I like your idea about environment variable placeholders in the configuration file. Let me do some research on what the likely environment of ssh-agent-mux would be when it is started by a user session launchd and also how/whether this might be useful on other platforms. I'll probably take to the end of the week to give that and your changes a good look, so bear with me!