feat(shell/lean): add --permissive command line option

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/server): add SET command

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(bin/lmake): support --permissive lean option

close #42
feat(emacs): use --permissive lmake option for flycheck

close #42