Add working_dir conf for tabs in projects

[Scaldabagno]

In a tmuxinator manner, I'd love to have in plugin's configuration a folder defined per tab and panes too. This because, inside a project, it could be necessary a multifolder configuration for reaching multiple components (e.g. frontend tab with its panes and backend tab with its panes)

tmuxinator example:

name: stuff
root: ~/

windows:
  - stuff1:
      root: ~/stuff1
      panes:
        - shell:
            -

  - stuff2:
      root: ~/stuff2
      panes:
        - shell:

herdr plus desidered configuration:

name = "Stuff"
description = "Stuff"
working_dir = "~/" # General, the default if not defined in tabs below

[[tabs]]
name = "Stuff 1"
working_dir = "~/stuff1"

[[tabs]]
name = "Stuff 2"
working_dir = "~/stuff2"