Add configuration options for ssh-agent and automatic cleanup on exit

[zazabe]

• Introduced SSH_AGENT_OPTS to allow users to pass additional parameters to ssh-agent.
• Implemented automatic stopping of ssh-agent on fish exit if started by the plugin.

[danhper]

Hi and thanks for the PR.
If you want to make the cleanup configurable, I'd be fine with that, but I personally don't want my ssh-agent to be killed if my fish process stops (even if it was started by this process)