Add support for TOML configuration files

This is used in many contexts, including being the format used by
Cargo, the Rust package manager.

testdata/expected/file.toml

@@ -0,0 +1,16 @@
+# Copyright 2018 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+[package]
+name = "not-a-real-crate"

testdata/initial/file.toml

@@ -0,0 +1,2 @@
+[package]
+name = "not-a-real-crate"