Implementing a Feature to easily set GDB breakpoint on Rop.Chain

This feature should enable user to easily set a gdb breakpoint in a `ROP.chain()` from a pwntools script.

Perhaps it could be implemented under the `rop` module.

Examples of possible successful implementation:

````py
binsh = next(libc.search(b'/bin/sh'))

rop = ROP(elf)
rop.raw("A"*72)
rop.system(binsh)

p.sendline(rop.chain())
gdb.attach(io, gdbscript='b* ' + rop[0])
```

Which would set a breakpoint at the start of the rop.chain.