Add unload target command

[PouuleT]

Fixes #17

[goscan] > load target SINGLE 192.168.1.1
[*] Imported target: 192.168.1.1
[goscan] > load target SINGLE 1.1.1.1
[*] Imported target: 1.1.1.1
[goscan] > show targets
+-------------+----------+
|   ADDRESS   |   STEP   |
+-------------+----------+
| 192.168.1.1 | IMPORTED |
+-------------+----------+
| 1.1.1.1     | IMPORTED |
+-------------+----------+
[goscan] > unload target SINGLE 192.168.1.1
[*] Target deleted
[goscan] > show targets
+---------+----------+
| ADDRESS |   STEP   |
+---------+----------+
| 1.1.1.1 | IMPORTED |
+---------+----------+
[goscan] > unload target SINGLE 8.8.8.8
[!] Error while deleting: No such target