Adds fix-yaml hook

This adds missing --- header to yaml file, which is required for
strict linting via yamllint. Future versions may add other fixes
for yaml files.

Fixes: #348

tests/fix_simple_yaml_test.py

@@ -0,0 +1,16 @@
+from __future__ import absolute_import
+from __future__ import unicode_literals
+
+from pre_commit_hooks import fix_yaml
+
+
+def test_no_change(tmpdir):
+    f = tmpdir.join('f.yaml')
+    f.write('---\nfoo: bar')
+    assert fix_yaml.main((f.strpath,)) == 0
+
+
+def test_change(tmpdir):
+    f = tmpdir.join('f.yaml')
+    f.write('foo: bar')
+    assert fix_yaml.main((f.strpath,)) == 1