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/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