Add flag for simple_prompt.

As this mode seem to be useful beyond testing, expose it via a flag and
make it official.

docs/source/whatsnew/pr/simple_prompt.rst

@@ -0,0 +1,4 b''
+when using IPython as a subprocess, like for emacs inferior-shell, IPython can
+be started with --simple-prompt flag, which will bypass the prompt_toolkit
+input layer. In this mode completion, prompt color and many other features are
+disabled.

IPython/terminal/ipapp.py

@@ -102,6 +102,11 b" addflag('autoedit-syntax', 'TerminalInteractiveShell.autoedit_syntax',"
         'Turn on auto editing of files with syntax errors.',
         'Turn off auto editing of files with syntax errors.'
 )
+addflag('simple-prompt', 'TerminalInteractiveShell.simple_prompt',
+        "Force simple minimal prompt using `raw_input`",
+        "Use a rich interactive prompt with prompt_toolkit",
+)
+
 addflag('banner', 'TerminalIPythonApp.display_banner',
         "Display a banner upon starting IPython.",
         "Don't display a banner upon starting IPython."

IPython/terminal/ptshell.py

@@ -31,6 +31,7 b' from .pt_inputhooks import get_inputhook_func'
 from .interactiveshell import get_default_editor, TerminalMagics
 from .ptutils import IPythonPTCompleter, IPythonPTLexer
 
+_use_simple_prompt = 'IPY_TEST_SIMPLE_PROMPT' in os.environ or not sys.stdin.isatty()
 
 class TerminalInteractiveShell(InteractiveShell):
     colors_force = True
@@ -46,6 +47,18 b' class TerminalInteractiveShell(InteractiveShell):'
     debugger_history = None
     debugger_cls = TerminalPdb
 
+    simple_prompt = Bool(_use_simple_prompt,
+        help="""Use `raw_input` for the REPL, without completion, multiline input, and prompt colors.
+
+            Useful when controlling IPython as a subprocess, and piping STDIN/OUT/ERR. Known usage are:
+            IPython own testing machinery, and emacs inferior-shell integration through elpy.
+
+            This mode default to `True` if the `IPY_TEST_SIMPLE_PROMPT`
+            environment variable is set, or the current terminal is not a tty.
+
+            """
+            ).tag(config=True)
+
     autoedit_syntax = Bool(False,
         help="auto editing of files with syntax errors.",
     ).tag(config=True)
@@ -117,7 +130,7 b' class TerminalInteractiveShell(InteractiveShell):'
         ]
 
     def init_prompt_toolkit_cli(self):
-        if ('IPY_TEST_SIMPLE_PROMPT' in os.environ) or not sys.stdin.isatty():
+        if self.simple_prompt:
             # Fall back to plain non-interactive output for tests.
             # This is very limited, and only accepts a single line.
             def prompt():