Fix doc tests without into-regex feature (#11, #10)

Author: ilslv

README.md

@@ -14,13 +14,16 @@ Rust implementation of [Cucumber Expressions].
 This crate provides [AST] parser, and [`Regex`] expansion of [Cucumber Expressions].
 
 ```rust
+# // `Regex` expansion requires `into-regex` feature.
+# #[cfg(feature = "into-regex")] {
 use cucumber_expressions::Expression;
 
 let re = Expression::regex("I have {int} cucumbers in my belly").unwrap();
 let caps = re.captures("I have 42 cucumbers in my belly").unwrap();
 
 assert_eq!(&caps[0], "I have 42 cucumbers in my belly");
 assert_eq!(&caps[1], "42");
+# }
 ```