emacs: remove auto revert mode

auto revert mode is dangerous, if the file in disk crash, there is no
way to restore.

Signed-off-by: Micky Ching <mickyching@163.com>
elpa: remove elpa from .gitignore

Signed-off-by: Micky Ching <mickyching@163.com>
elpa: add elpa plugins

Signed-off-by: Micky Ching <mickyching@163.com>