magic_arguments: dedent but otherwise preserve indentation.

Previously magic_arguments stripped all whitespace at the beginning of
each line, interfering with formatting syntax which relies on indentation
to give context (e.g., code blocks). Now the docstring text is passed
through dedent to strip the global indentation before being handed off
to RawDescriptionHelpFormatter which preserves any remaining indentation.

Thanks to @rkern for suggesting the solution approach.

Closes gh-1890.

IPython/core/magic_arguments.py

@@ -50,7 +50,13 b' arguments::'
 from IPython.external import argparse
 from IPython.core.error import UsageError
 from IPython.utils.process import arg_split
+from IPython.utils.text import dedent
 
+class MagicHelpFormatter(argparse.RawDescriptionHelpFormatter):
+    """ A HelpFormatter which dedents but otherwise preserves indentation.
+    """
+    def _fill_text(self, text, width, indent):
+        return argparse.RawDescriptionHelpFormatter._fill_text(self, dedent(text), width, indent)
 
 class MagicArgumentParser(argparse.ArgumentParser):
     """ An ArgumentParser tweaked for use by IPython magics.
@@ -62,7 +68,7 b' class MagicArgumentParser(argparse.ArgumentParser):'
                  epilog=None,
                  version=None,
                  parents=None,
-                 formatter_class=argparse.HelpFormatter,
+                 formatter_class=MagicHelpFormatter,
                  prefix_chars='-',
                  argument_default=None,
                  conflict_handler='error',