Make the built-in snippets more extensible

The commit opens up all of the default snippets so that it's easier
to extend them and modify their behaviour. All of the built-in
snippets are now public with protected constructors. Also, where
appropriate the models used by the snippets have been made more
fine-grained.

Closes gh-73

spring-restdocs-core/src/test/resources/custom-snippet-templates/curl-request-with-title.snippet

@@ -1,5 +1,5 @@
 [source,bash]
 .{{title}}
 ----
-$ curl {{arguments}}
+$ curl {{url}} {{options}}
 ----