Allow preprocessing to be applied to every test

Following the reworking of the API, it was no longer possible to
apply preprocessing to every test and to still customize the snippets
used by that test.

This commit adds a new snippets() method to
RestDocumentationResultHandler that allows additional snippets to be
configured once the result handler has been created. The documentation
has been updated to describe how to apply preprocessing to every
test and Spring HATEOAS sample has been updated to illustrate the
approach.

Closes gh-88

samples/rest-notes-spring-hateoas/src/main/resources/application.properties

@@ -1 +0,0 @@
-spring.jackson.serialization.indent_output: true