Add org-mode agenda config for GTD

 - Change variable `org-todo-keywords'
  - Add 'NEXT'
  - Replace 'STARTED' with 'DOING'
  - Change keybind
 - Change variable `org-tag-alist'
  - Add 'BIGROCK'
  - Change action tags with uppercase
 - Set variable `org-stuck-projects'
 - Set variable `org-agenda-custom-commands'
  - Add Next Action list
 - Set variable `org-refile-targets'
Add org-mode export config