radare2 integration via RAP protocol

[jroimartin]

This PR adds a custom command called rap that implements a radare RAP server.

To start using it, add the following module to pyrebox.conf:

[MODULES]
radare2.rap: True

Then, from the pyrebox shell, use the following command to start a RAP server listening on localhost:1234:

[1] pyrebox> custom rap :1234

After that, you can connect to pyrebox using r2 with:

$ r2 -e 'io.va=0' -w rap://localhost:1234/<pid or proc name>

The RAP server can be closed from the pyrebox shell with:

[2] pyrebox> custom rap shutdown