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.

testing/fixtures.py

@@ -123,6 +123,14 @@ def add_config_to_repo(git_path, config, config_file=C.CONFIG_FILE):
     return git_path
 
 
+def remove_config_from_repo(git_path, config_file=C.CONFIG_FILE):
+    os.unlink(os.path.join(git_path, config_file))
+    with cwd(git_path):
+        cmd_output('git', 'add', config_file)
+        cmd_output('git', 'commit', '-m', 'Remove hooks config')
+    return git_path
+
+
 def make_consuming_repo(tempdir_factory, repo_source):
     path = make_repo(tempdir_factory, repo_source)
     config = make_config_from_repo(path)