feat(emacs/README.md): add instr. for MELPA; add whitespace-cleanup-mode
fix(emacs): use cl-case, require cl-lib
fix(library/standard): remove long comments introduced by b2c2d1d
feat(emacs): use optional package when required
feat(emacs/lean-setting.el): add lean-rule-color custom-var
feat(emacs): provide an option to disable lean-flycheck
feat(emacs): add lean-flycheck-turn-on/off/toggle