Add auto install feature for pre-commit plugin

[marcodisarno]

Standards checklist:

This was inspired by ohmyzsh's dotenv plugin, which I've grown to like a lot. I re-used the code from it's source_env function, so thanks to whoever wrote that!

• 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:

• adds a zsh chpwd hook to automatically prompt for installing pre-commit's git hooks when .pre-commit-config.yaml exists in the current working dir.

Other comments:

it's quite configurable, but switched to "Prompt" by default.