Alternative configuration support

Following #127, we'll probably want to support configuration from sources other than tmux.conf.
This is to track adding support for an alternative configuration source -- probably something simple like ff's plain config.

Look at issue #127 to understand the current tmux.conf configuration. Examine the codebase for how configuration is currently loaded, likely in a main.go or config package. Determine how to add support for a plain config file format, possibly JSON or YAML. Check for existing tests around configuration to understand the expected behavior.

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