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.

testing/resources/logfile_repo/.pre-commit-hooks.yaml

@@ -0,0 +1,6 @@
+-   id: logfile test hook
+    name: Logfile test hook
+    entry: bin/hook.sh
+    language: script
+    files: .
+    log_file: test.log

testing/resources/logfile_repo/bin/hook.sh

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+echo "This is STDOUT output"
+echo "This is STDERR output" 1>&2
+
+exit 1