Add stdin support for reading policy

* Add ability to read from  stdin for a policy to be analyzed such as
  `cat file.json | parliament`
* Adds new FileType argument to read from stdin and don't allow both
  --file and stdin
* Implements duo-labs#163

parliament/cli.py

@@ -138,6 +138,11 @@ def main():
         type=str,
     )
     parser.add_argument("--file", help="Provide a policy in a file", type=str)
+    parser.add_argument('--stdinfile',
+                            help="Provide a policy via stdin instead of --file",
+                            nargs='?',
+                            type=argparse.FileType('r'),
+                            default=sys.stdin)
     parser.add_argument(
         "--directory", help="Provide a path to directory with policy files", type=str
     )
@@ -212,6 +217,10 @@ def main():
     if args.minimal and args.json:
         raise Exception("You cannot choose both minimal and json output")
 
+    # If I have some stdin to read it should be my policy so don't allow file input
+    if not sys.stdin.isatty() and args.file:
+        parser.error("You cannot pass a file with --file and use stdin together")
+
     # Change the exit status if there are errors
     exit_status = 0
     findings = []
@@ -321,6 +330,17 @@ def main():
                 config=config,
             )
             findings.extend(policy.findings)
+    elif not sys.stdin.isatty():
+        contents = args.stdinfile.read()
+        args.stdinfile.close()
+        policy = analyze_policy_string(
+            contents,
+            args.file,
+            private_auditors_custom_path=args.private_auditors,
+            include_community_auditors=args.include_community_auditors,
+            config=config,
+        )
+        findings.extend(policy.findings)
     elif args.directory:
         file_paths = find_files(
             directory=args.directory,