cli: add option for changing save path

cli: add option for changing save path

Save path is a directory where config change (delta) files are stored.
Having a custom individual save dir can be used to prevent two (or more)
"uci" cli callers (e.g. bash scripts) from commiting each other changes.

In the following example:

App0					App1
----					----
uci set system.@system[0].timezone=UTC
					uci set system.@system[0].hostname=OpenWrt
					uci commit system

App1 would unintentionally commit changes made by App0. This can be
avoided by at least 1 "uci" cli user specifying a custom -t option.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>