Skip compressed files

This prevents spurious UnicodeDecodeErrors in Python 3.

Adding handling for compressed files would not be hard (using gzip, bz2,
and optionally lzma libraries), but there's probably little benefit,
since compressed files in a project tree are likely either from an
upstream source or have an uncompressed version available for testing.

dodgy/run.py

@@ -40,6 +40,9 @@ def run_checks(directory, ignore_paths=None):
         mimetype = mimetypes.guess_type(filepath)
         if mimetype[0] is None or not mimetype[0].startswith('text/'):
             continue
+        # Also skip anything with an encoding (e.g., a gzipped CSS).
+        if mimetype[1]:
+            continue
 
         for msg_parts in check_file(filepath):
             warnings.append({