Support @generated marker to skip code formatting

`@generated` marker is used by certain tools to understand that the
file is generated, so it should be treated differently than a file
written by a human:
* linters should not be invoked on these files,
* diffs in these files are less important,
* and these files should not be reformatted.

This PR proposes builtin support for `@generated` marker.

I have not found a standard for a generated file marker, but:
* Facebook [uses `@generated` marker](https://tinyurl.com/fb-generated)
* Phabricator tool which was spawned from Facebook internal tool
  [also understands `@generated` marker](https://git.io/JnVHa)
* Cargo inserts `@generated` marker into [generated Cargo.lock files](https://git.io/JnVHP)

My personal story is that rust-protobuf project which I maintain
was broken twice because of incompatibilities/bugs in rustfmt marker handling:
[one](stepancheg/rust-protobuf#493),
[two](stepancheg/rust-protobuf#551).

While rustfmt AST markers are useful to apply to a certain AST
elements, disable whole-file-at-once all-tools-at-once text level
marker might be easier to use and more reliable for generated code.

src/formatting.rs

@@ -146,6 +146,12 @@ impl<'a, T: FormatHandler + 'a> FormatContext<'a, T> {
         is_macro_def: bool,
     ) -> Result<(), ErrorKind> {
         let snippet_provider = self.parse_session.snippet_provider(module.span);
+
+        let generated_marker = concat!("@", "generated");
+        if snippet_provider.entire_snippet().contains(generated_marker) {
+            return Ok(());
+        }
+
         let mut visitor = FmtVisitor::from_parse_sess(
             &self.parse_session,
             &self.config,

tests/source/generated.rs

@@ -0,0 +1,14 @@
+fn
+
+foo
+
+(
+    x: u32
+) {
+
+
+
+
+}
+
+// This file is @generated

tests/target/generated.rs

@@ -0,0 +1,14 @@
+fn
+
+foo
+
+(
+    x: u32
+) {
+
+
+
+
+}
+
+// This file is @generated