Add fmtstringpayload command

I've written a new command able to create the payload for format string bug exploitation.
It take at least 3 parameters : what write, where write and the number of formater (BBBB%4$x - 4 is the number of formater). It can also take the number of chars already printed.

Example, to write 0x1337babe at 0x08048425, you have to do : 
gdb-peda$ fmtstringpayload 0x1337babe 0x08048425 7