Make line_input_checker=True the default for InputSplitter.

The Qt console needs to use it in line_input_checker mode, but can't specify
arguments in case it's using the InputSplitter base class.

Closes gh-3248

IPython/core/inputsplitter.py

@@ -477,7 +477,7 b' class IPythonInputSplitter(InputSplitter):'
     # List with lines of raw input accumulated so far.
     _buffer_raw = None
 
-    def __init__(self, line_input_checker=False, physical_line_transforms=None,
+    def __init__(self, line_input_checker=True, physical_line_transforms=None,
                     logical_line_transforms=None, python_line_transforms=None):
         super(IPythonInputSplitter, self).__init__()
         self._buffer_raw = []