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), []),
 )

pre_commit/commands/run.py

@@ -121,7 +121,10 @@ def _run_single_hook(hook, repo, args, skips, cols):
         for out in (stdout, stderr):
             assert type(out) is bytes, type(out)
             if out.strip():
-                output.write_line(out.strip())
+                output.write_line(
+                    out.strip(),
+                    logfile_name=hook.get('log_file'),
+                )
         output.write_line()
 
     return retcode

pre_commit/output.py

@@ -71,8 +71,17 @@ def write(s, stream=stdout_byte_stream):
     stream.flush()
 
 
-def write_line(s=None, stream=stdout_byte_stream):
-    if s is not None:
-        stream.write(five.to_bytes(s))
-    stream.write(b'\n')
-    stream.flush()
+def write_line(s=None, stream=stdout_byte_stream, logfile_name=None):
+    def output_streams():
+        yield stream
+        try:
+            with open(logfile_name, 'ab') as logfile:
+                yield logfile
+        except (TypeError, IOError):
+            pass
+
+    for output_stream in output_streams():
+        if s is not None:
+            output_stream.write(five.to_bytes(s))
+        output_stream.write(b'\n')
+        output_stream.flush()