Add --allow-missing-config option to install

When no '.pre-commit-config.yaml' file exists while `pre-commit` hooks
are enabled, `pre-commit` returns an error and the action is aborted.
This is a very common scenario when pre-commit is added later on a
project and the user wants to work on a previous branch where the
configuration file does not exist.

This commits allow the user to optionally install the `pre-commit` hooks
with an option to allow a missing configuration and trigger only the
legacy pre-commit hooks (if any) when it is missing.

pre_commit/resources/hook-tmpl

@@ -10,6 +10,7 @@ retv=0
 args=""
 
 ENV_PYTHON='{sys_executable}'
+SKIP_ON_MISSING_CONF={skip_on_missing_conf}
 
 which pre-commit >& /dev/null
 WHICH_RETV=$?
@@ -37,6 +38,20 @@ if [ -x "$HERE"/{hook_type}.legacy ]; then
     fi
 fi
 
+CONF_FILE=$(git rev-parse --show-toplevel)"/.pre-commit-config.yaml"
+if [ ! -f $CONF_FILE  ]; then
+    if [ $SKIP_ON_MISSING_CONF = true ] || [ ! -z $PRE_COMMIT_ALLOW_NO_CONFIG ]; then
+            echo '`.pre-commit-config.yaml` config file not found. Skipping `pre-commit`.'
+            exit $retv
+    else
+        echo 'No .pre-commit-config.yaml file was found\n'\
+             '- To temporarily silence this, run `PRE_COMMIT_ALLOW_NO_CONFIG=1 git ...`\n'\
+             '- To permanently silence this, install pre-commit with the `--allow-missing-config` option\n'\
+             '- To uninstall pre-commit run `pre-commit uninstall`'
+        exit 1
+	fi
+fi
+
 {pre_push}
 
 # Run pre-commit