Summary of commit:

Changes:
* required.el:
  - Add undo-tree
* keybindings.el:
  - Add keybindings for hiding and showing code blocks
  - Replace god-local-mode to god-mode for "i" keybinding
* enabled.el:
  - Add global-undo-tree-mode
* auto-install.el:
  - Add undo-tree
* README.md:
  - Replace space to SPC
  - Replace BACKSPACE to DEL
  - Add instructions for manipulating text