fix(frontends/lean): missing ':' in error messages

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(emacs): add short-cuts \C-c\C-l and \C-c\C-x to execute Lean files in Emacs

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(emacs): include lean-mode Emacs files in the distribution

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(util/format): do not use colors by default

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>