ENH: Accept one-off exception mode setting for %tb.

This makes it easy to access a more verbose representation that the
currently-active exception reporting mode provides.

```python
In [1]: %xmode minimal
Exception reporting mode: Minimal

In [2]: def f():
   ...:     3/0
   ...:

In [3]: f()
ZeroDivisionError: division by zero

In [4]: %tb verbose
---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-3-c43e34e6d405> in <module>
----> 1 f()
        global f = <function f at 0x10e52de18>

<ipython-input-2-cd4172d70b5b> in f()
      1 def f():
----> 2     3/0
      3

ZeroDivisionError: division by zero

In [5]: f()  # The default reporting mode has not been changed....
ZeroDivisionError: division by zero
```

IPython/core/magics/execution.py

@@ -466,10 +466,35 b' python-profiler package from non-free.""")'
 
     @line_magic
     def tb(self, s):
-        """Print the last traceback with the currently active exception mode.
+        """Print the last traceback.
 
-        See %xmode for changing exception reporting modes."""
-        self.shell.showtraceback()
+        Optionally, specify an exception reporting mode, tuning the
+        verbosity of the traceback. By default the currently-active exception
+        mode is used. See %xmode for changing exception reporting modes.
+
+        Valid modes: Plain, Context, Verbose, and Minimal.
+        """
+        interactive_tb = self.shell.InteractiveTB
+        if s:
+            # Switch exception reporting mode for this one call.
+            # Ensure it is switched back.
+            def xmode_switch_err(name):
+                warn('Error changing %s exception modes.\n%s' %
+                    (name,sys.exc_info()[1]))
+
+            new_mode = s.strip().capitalize()
+            original_mode = interactive_tb.mode
+            try:
+                try:
+                    interactive_tb.set_mode(mode=new_mode)
+                except Exception:
+                    xmode_switch_err('user')
+                else:
+                    self.shell.showtraceback()
+            finally:
+                interactive_tb.set_mode(mode=original_mode)
+        else:
+            self.shell.showtraceback()
 
     @skip_doctest
     @line_magic