Add support for placeholder resolution in stub file names

E.g. document("{method-name}/{step}").

Fixes gh-84

samples/standalone/restdocs/http-server/src/test/java/com/example/fraud/StubGeneratorTests.java

@@ -56,7 +56,7 @@ public class StubGeneratorTests {
 				.andDo(verify().jsonPath("$.clientId")
 						.jsonPath("$[?(@.loanAmount > 1000)]")
 						.contentType(MediaType.valueOf("application/vnd.fraud.v1+json"))
-						.stub("markClientAsFraud"));
+						.stub("markClientAsFraud/{step}"));
 	}
 
 	@Test