Add support for piping to stdout

Not sure if this is actually the desired behavior, but this
will allow for things like:

  generate-json | jpterm | process-result-json

Once you find an expression you like you can just hit
"ctrl r" and once you exit it will pipe the result to the
next command.

Another option is just to support writing the output of the command
to a file.

jpterm.py

@@ -64,6 +64,8 @@ def __init__(self, input_data):
         self.lexer = pygments.lexers.get_lexer_by_name('json')
         self.formatter = ConsoleJSONFormatter()
         self.saved_expressions = []
+        self._latest_search_result = None
+        self._last_saved_search_result = None
 
     def _create_colorized_json(self, json_string):
         tokens = self.lexer.get_tokens(json_string)
@@ -119,6 +121,7 @@ def _on_edit(self, widget, text):
             return
         try:
             result = jmespath.compile(text).search(self.parsed_json)
+            self._latest_search_result = result
             self.footer.set_text("Status: success")
         except Exception:
             pass
@@ -149,10 +152,18 @@ def unhandled_input(self, key):
         elif key == 'ctrl p':
             self.saved_expressions.append(self.input_expr.edit_text)
             self.footer.set_text("Status: expression saved")
+        elif key == 'ctrl r':
+            if self._latest_search_result is not None:
+                self._last_saved_search_result = self._latest_search_result
+                self.footer.set_text("Status: result saved")
 
     def display_saved_expressions(self):
         for expression in self.saved_expressions:
-            print(expression)
+            sys.__stdout__.write(expression)
+            sys.__stdout__.write('\n')
+        if self._last_saved_search_result is not None:
+            sys.__stdout__.write(json.dumps(self._last_saved_search_result, indent=2))
+            sys.__stdout__.write('\n')
 
 
 def _load_input_json(filename):
@@ -173,6 +184,14 @@ def _load_input_json(filename):
     return input_json
 
 
+def switch_out_stdout():
+    if not os.isatty(sys.stdout.fileno()):
+        # If stdout is a pipe, then swap out the current stdout
+        # for the controlling tty.  That way we can pipe
+        # the output of this command.
+        sys.stdout = open(os.ctermid(), 'w')
+
+
 def main():
     parser = argparse.ArgumentParser(description=__doc__)
     parser.add_argument('input-json', nargs='?',
@@ -189,6 +208,7 @@ def main():
         sys.stderr.write("Unable to load the input JSON: %s\n\n" % e)
         return 1
 
+    switch_out_stdout()
     screen = urwid.raw_display.Screen()
     display = JMESPathDisplay(input_json)
     display.main(screen=screen)