add ack prefix map under search prefix map.
update workgroups2 mode-line format.
update workgroups2 string on mode line.
add keybinding for helm-moccur/helm-multi-occur.
add helm-mu and configure it.
do not let workgroups2 display its string. use my custom in mode-line-format instead.
re-sort helm extensions in el-get packages list.
migrate helm-mu configuration to mu4e init file.

Because helm is loaded at first before my custom keybindings settings.
So I put it in mu4e init file.