Add support for shell commands in environment variables

[dagon666]

I've prepared a change to add support for environment variables values resolved using a shell command.

Within and environment block, the variable can be defined using a literal or with a shell_command key which contains the command used to generate the value e.g.:

environment:  
  FOO:
    shell_command: "echo VALUE_FOR_FOO"
  BAR: "literal_values_work_the_old_way"

This is useful when injecting API keys from a password manager:

environment:  
  MYAPI_KEY:
    shell_command: "pass show MYAPI_KEY"

I've already prepared a branch with a change: https://github.com/dagon666/tmuxp/tree/feat/env-cmd