Enable recursive .env sourcing

[heitorpolidoro]

Standards checklist:

• The PR title is descriptive.
• The PR doesn't replicate another PR which is already open.
• I have read the contribution guide and followed all the instructions.
• The code follows the code style guide detailed in the wiki.
• The code is mine or it's from somewhere with an MIT-compatible license.
• The code is efficient, to the best of my ability, and does not waste computer resources.
• The code is stable and I have tested it myself, to the best of my abilities.
• If the code introduces new aliases, I provide a valid use case for all plugin users down below.

Changes:

• Add an option to do a recursive source of .env, from a configured ROOT directory to the current working directory, enabling ENVs overwriting
• ENVs added
  • ZSH_DOTENV_RECURSIVE: Default false, if true enables recursive sourcing
  • ZSH_DOTENV_ROOT: Default $HOME, the root directory to starting sourcing from

Other comments:

My use case: I need to install a python library from a private repository in more than one os my repositories so, instead off heaving multiples .env to define GH_USERNAME and GH_TOKEN I can have only one.

@mcornella