Add support for passing values to start command

## Overview It would be nice to be able to reference variables in templates and pass them in when starting a project. This makes the templates more flexible and reusable. ## Detailed Description In my workflow I like to have a default template that I define and whenever I go to create a new session in tmux I check to see if a specific template exists for the project or fallback to spinning up the default. To make this work I would need the root and session name to be dynamic and defined at runtime. Another use case I can see is a project that is using git worktrees where a lot of the project layout is shared but I might want to pass in args to spin up several versions of that template. ## Benefits You can write less templates by having more dynamic templates. It also makes it possible to have laio be your sole session creator and not juggle multiple session creation tools. ## Alternatives For the bulk of my use case being able to define a default template (_default.yaml?) that is spun up when passing an unrecognized project name would work. Id have to work around things to get git worktrees working. ## Additional Context Tmuxinator has this ability https://github.com/tmuxinator/tmuxinator?tab=readme-ov-file#erb and so does my current tool smug https://github.com/ivaaaan/smug?tab=readme-ov-file#custom-settings (I requested the feature be added). But I am liking the idea of layouts that laio has and want to give it a go but this would be a blocker.