Think about running the wire/cable/string/rope through 3 pulleys. the 2 outer ones are fixed and the one in the middle is on a spring loaded arm and floats, depending on cable tension. You connect that to a potentiometer or several switches. one switch for tight, one for feeding nicely, and 1 for way loose. When the cable tight is on the cable can be fed. WHen the cable is in the feeding nicely position you can stop the motor until cable tight, and on cable loose you wind in the cable. with a potentiometer you could use the position to control the feed in and out so you could control the tension.