Adds support for 'log_file' in hook config

Specify a filename on a per hook basis and
pre-commit will write the STDOUT and STDERR
of that hook into the file. Useful for CI.

Resolves #499.

pre_commit/clientlib.py

@@ -53,6 +53,7 @@ MANIFEST_HOOK_DICT = schema.Map(
         '^$',
     ),
     schema.Optional('language_version', schema.check_string, 'default'),
+    schema.OptionalNoDefault('log_file', schema.check_string),
     schema.Optional('minimum_pre_commit_version', schema.check_string, '0'),
     schema.Optional('stages', schema.check_array(schema.check_string), []),
 )