adding simple r2 (radare) command with auto-seek to $pc (#188)

It's quite handy to drop into radare2 in the middle of a deep debugging
session to fire up the visual mode and examine the current location in
gdb using the ascii graph view (or something else) in radare2.

pwndbg/__init__.py

@@ -33,6 +33,7 @@ import pwndbg.commands.misc
 import pwndbg.commands.next
 import pwndbg.commands.peda
 import pwndbg.commands.procinfo
+import pwndbg.commands.radare2
 import pwndbg.commands.reload
 import pwndbg.commands.rop
 import pwndbg.commands.ropper

pwndbg/commands/radare2.py

@@ -0,0 +1,37 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+import argparse
+import subprocess
+
+import pwndbg.commands
+
+parser = argparse.ArgumentParser(description=".",
+                                 epilog="Example: r2 -- -S -AA")
+parser.add_argument('--no-seek', action='store_true',
+                    help='Do not seek to current pc')
+parser.add_argument('arguments', nargs='*', type=str,
+                    help='Arguments to pass to radare')
+
+@pwndbg.commands.ArgparsedCommand(parser)
+def r2(arguments, no_seek=False):
+    filename = pwndbg.file.get_file(pwndbg.proc.exe)
+
+    if not filename:
+        print('No file is selected')
+        return
+
+    # Build up the command line to run
+    cmd = ['radare2', filename]
+    if not no_seek and pwndbg.proc.alive:
+        cmd.extend(['-s', hex(pwndbg.regs.pc)])
+    cmd += arguments
+
+    try:
+        subprocess.call(cmd)
+    except Exception:
+        print("Could not run radare2.  Please ensure it's installed and in $PATH.")