Isearch should store aborted search strings too

[Tom <adatgyujto@gmail.com>]

I often use isearch to check something in other parts of the
file and then go back to the starting point by aborting isearch
with ctrl+G.

When I abort it then isearch doesn't store the search string which
I often need again, so I have to type it again. I recommend
adding and option to control this behavior and store the search string 
even if aborting.

This could even be the default, since previous search strings can
be trivially accessed with M-p, so it's no problem if more
strings are stored than before.