Enhancements

1. Enhance go-mode.
2. Enhance popwin.
3. Enhance org-mode.
4. Remove org-bullets due to bad performance.
5. Add magithub.
6. Preview Markdown with github style
7. Use highlight-indent-guides' instead of indent-guide'.
8. Improve keybindings for ag', pt', `rg'.
9. Improve company and flycheck.
10. Add command: update-dotfiles.
11. Fix: dired doesn't work without gls on macOS.
12. Fix: #16 remove excessive quote to fix python mode problem.
13. Remove highlight-parentheses.