lint: Add pep8 checker to the project.

We set this up initially with all of the rules that Zulip violates
disabled.

Also, the pep8 linter is substantially slower than the other Zulip
linters, so we've put it behind an option to `tools/lint-all`.

requirements/common.txt

@@ -165,3 +165,6 @@ git+https://github.com/umairwaheed/virtualenv-clone.git@short-version#egg=virtua
 
 # Needed for reading json as stream
 ijson==2.3
+
+#for pep8 linter
+pycodestyle==2.1.0

tools/lint-all

@@ -77,6 +77,27 @@ def check_pyflakes(options, by_lang):
                 failed = True
     return failed
 
+
+def check_pep8(files):
+    # type: (List[str]) -> bool
+    failed = False
+    ignored_rules = [
+        'E402', 'E501', 'W503', 'E711', 'E201', 'E203', 'E202', 'E128', 'E226', 'E124', 'E125',
+        'E126', 'E127', 'E121', 'E122', 'E123', 'E266', 'E265', 'E261', 'E301', 'E221', 'E303',
+        'E241', 'E712', 'E225', 'E401', 'E115', 'E114', 'E111', 'E222', 'E731', 'E302', 'E129',
+        'E741', 'E714', 'W391', 'E211', 'E713', 'E502', 'E131', 'E305', 'E251', 'E306', 'E231',
+        'E701', 'E702', 'E703',
+    ]
+    pep8 = subprocess.Popen(
+        ['pycodestyle'] + files + ['--ignore={rules}'.format(rules=','.join(ignored_rules))],
+        stdout=subprocess.PIPE, stderr=subprocess.PIPE, universal_newlines=True)
+    for pipe in (pep8.stdout, pep8.stderr):
+        for ln in pipe:
+            sys.stdout.write(ln)
+            failed = True
+    return failed
+
+
 def run_parallel(lint_functions):
     # type: (Dict[str, Callable[[], int]]) -> bool
     pids = []
@@ -397,6 +418,9 @@ def run():
     parser.add_option('--full',
         action='store_true',
         help='Check some things we typically ignore')
+    parser.add_option('--pep8',
+        action='store_true',
+        help='Run the pep8 checker')
     parser.add_option('--modified', '-m',
         action='store_true',
         help='Only check modified files')
@@ -486,6 +510,13 @@ def run():
             failed = check_pyflakes(options, by_lang)
             return 1 if failed else 0
 
+        if options.pep8:
+            @lint
+            def pep8():
+                # type: () -> int
+                failed = check_pep8(by_lang['py'])
+                return 1 if failed else 0
+
         failed = run_parallel(lint_functions)
 
     sys.exit(1 if failed else 0)