Support radare2 as a backend

[XVilka]

It can remove the need for LIEF, capstone, and ROPgadget at once.

See simple examples on how to use radare2 with ROP:

• http://www.radiofreerobotron.net/blog/2017/11/23/ropemporium-pivot-ctf-walkthrough/
• Simple search within radare2
• Exploitation Radare2 Project

And ROP search options https://r2wiki.readthedocs.io/en/latest/options/e/values-that-e-can-modify/rop/

[Boyan-MILANOV]

Hey !

That could be a nice feature indeed. Since I currently don't have so much time to work on ropgenerator I can not implement it immediately though.
Of course if you are familiar with radar2 feel free to submit a PR that I'll gladly welcome ;)