Feature: Add interactive git attributes generator

[sandr01d]

Check list

• I have performed a self-review of my code
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation

Description

Adds an interactive .gitattributes generator that works exactly the same as the one we already have for .gitignore. I've refactored the functions used in _forgit_ignore(), so they can also be used for _forgit_attributes(). When testing, it's a good idea to also test that gi still works as it did before.
Marked this as a draft, because I first want to sort out what to do with #407 and refactor _forgit_ignore_clean if necessary.

Closes #405

Type of change

• Bug fix
• New feature
• Refactor
• Breaking change
• Documentation change

Test environment

• Shell
  • bash
  • zsh
  • fish
• OS
  • Linux
  • Mac OS X
  • Windows
  • Others: