Modify stdin retriever to detect coding pragma

When we read in a file on Python 3, we use the tokenize module to detect
the coding pragma at the top of the file. We then use that to decode the
rest of the file. However, when we were receiving stdin, we would not do
that.

This updates ``stdin_get_value`` in a backwards compatible way to check
for that coding pragma and then fall back to UTF-8 if necessary.

Closes #306

src/flake8/utils.py

@@ -7,6 +7,7 @@ import os
 import platform
 import re
 import sys
+import tokenize
 
 DIFF_HUNK_REGEXP = re.compile(r'^@@ -\d+(?:,\d+)? \+(\d+)(?:,(\d+))? @@.*$')
 
@@ -65,17 +66,26 @@ def normalize_path(path, parent=os.curdir):
     return path.rstrip(separator + alternate_separator)
 
 
+def _stdin_get_value_py3():
+    stdin_value = sys.stdin.buffer.read()
+    fd = io.BytesIO(stdin_value)
+    try:
+        (coding, lines) = tokenize.detect_encoding(fd.readline)
+        return io.StringIO(stdin_value.decode(coding))
+    except (LookupError, SyntaxError, UnicodeError):
+        return io.StringIO(stdin_value.decode('utf-8'))
+
+
 def stdin_get_value():
     # type: () -> str
     """Get and cache it so plugins can use it."""
     cached_value = getattr(stdin_get_value, 'cached_stdin', None)
     if cached_value is None:
-        stdin_value = sys.stdin.read()
         if sys.version_info < (3, 0):
-            cached_type = io.BytesIO
+            stdin_value = io.BytesIO(sys.stdin.read())
         else:
-            cached_type = io.StringIO
-        stdin_get_value.cached_stdin = cached_type(stdin_value)
+            stdin_value = _stdin_get_value_py3()
+        stdin_get_value.cached_stdin = stdin_value
         cached_value = stdin_get_value.cached_stdin
     return cached_value.getvalue()