Add proposal for + syntax for inline snapshots

Quick demo; took 10 min. Would need a substantial discussion, docs, etc.

In particular, older versions of `cargo-insta` would have surprising behavior.

But I think it's a good idea; allows for `cargo fmt` to work on the inline snapshots.

cargo-insta/src/inline.rs

@@ -188,7 +188,8 @@ impl FilePatcher {
                 }
 
                 match tokens[tokens.len() - 2] {
-                    TokenTree::Punct(ref punct) if punct.as_char() == '@' => {}
+                    TokenTree::Punct(ref punct)
+                        if punct.as_char() == '@' || punct.as_char() == '+' => {}
                     _ => {
                         return false;
                     }

insta/src/macros.rs

@@ -330,6 +330,15 @@ macro_rules! _assert_snapshot_base {
             $($arg),*
         )
     };
+    // Or using the new syntax
+    (transform=$transform:expr, $($arg:expr),*, !$snapshot:literal $(,)?) => {
+        $crate::_assert_snapshot_base!(
+            transform = $transform,
+            #[allow(clippy::needless_raw_string_hashes)]
+            $crate::_macro_support::ReferenceValue::Inline($snapshot),
+            $($arg),*
+        )
+    };
     // If there's no debug_expr, use the stringified value, call self.
     (transform=$transform:expr, $name:expr, $value:expr $(,)?) => {
         $crate::_assert_snapshot_base!(transform = $transform, $name, $value, stringify!($value))

insta/tests/test_inline.rs

@@ -37,6 +37,16 @@ fn test_single_line() {
     assert_snapshot!("Testing", @"Testing");
 }
 
+#[test]
+fn test_single_line_plus() {
+    // New `+` syntax
+    assert_snapshot!("Testing", !"Testing");
+
+    // TODO: move to integration test: confirm this fails (it wouldn't fail with
+    // a `-` prefix, for example)
+    // assert_snapshot!("Testing", *5);
+}
+
 // We used to use the thread name for snapshot name detection.  This is unreliable
 // so this test now basically does exactly the same as `test_unnamed_single_line`.
 #[test]