Adds instructions for installing with brew

Fixes URL for homebrew release

Cleans up goreleaser config and adds Homebrew release

Adds version information

Adds documentation for the --only-sign flag

Makes ssh hostname positional argument optional if --only-sign flag is given since this flag indicates a desire to not connect to a host

Adds logic for only signing certificates and not executing ssh process

Adds --only-sign flag to support signing certificates without executing the ssh process

Adds goreleaser config

Adds VSSH prefix to environment variables

Adds demo gif to repository

Updates README

Adds documentation for the cmd package

Adds documentation for ssh package

Adds documentation to the ui package