Fixes #53 #750

Options like sleep: 3 and enter: false should be allowed to be passed at the per-command level.

Internally, individual tmuxp commands are only strings.

Now:

windows:
  panes:
  - shell_command:
    - echo "hello"

What the syntax needs:

windows:
  panes:
  - shell_command:
    - cmd: echo "hello"
      enter: false
      sleep: 2

Earlier conversations: https://github.com/tmux-python/tmuxp/issues/53#issuecomment-1066092004