Add command.variables as a convenience utility for defining global command variables

[DannyBen]

cc #562, #366

This PR adds command.variables array, which allows defining global variables that will be available when a command (or the root script) is executed.

• Each item in the array accepts a name, and a value.
• The value can be anything:
  • Strings will be quoted: thevar="thevalue"
  • Arrays will be declared: declare -a thevar=(the values)
  • Associative arrays will be declared as such: declare -A thevar=(["key"]="value"...)
  • Anything else (booleans, numbers) will be placed as is: thevar=thevalue

    variables:
    # Simple value
    - name: output_folder
      value: output

    # Array
    - name: download_sources
      value:
      - youtube
      - instagram

    # Associative array
    - name: zip_options
      value:
        pattern: "*.json"
        compression_level: fast

An appropriate example was added, and running bashly doc command.variables will show the appropriate in-terminal help.