Allow disabling all checks for a file

Allow disabling of a file, even if it is invalid YAML (syntax error) by
including `# yamllint disable-file` in the first line.

yamllint/linter.py

@@ -189,6 +189,10 @@ def _run(buffer, conf, filepath):
     assert hasattr(buffer, '__getitem__'), \
         '_run() argument must be a buffer, not a stream'
 
+    first_line = next(parser.line_generator(buffer)).content
+    if re.match(r'^#\s*yamllint disable-file\s*$', first_line):
+        return
+
     # If the document contains a syntax error, save it and yield it at the
     # right line
     syntax_error = get_syntax_error(buffer)