Add support for el_no_hist to disable access to and auto-save of history.

This commit adds a new global variable 'el_no_hist' which can be used
to disable auto-save of history as well as access to history using prev
and next keybindings (up/down arrows).

Signed-off-by: Mattias Walström <lazzer@gmail.com>
Signed-off-by: Joachim Nilsson <troglobit@gmail.com>