This commit introduces a new TemplateEngine abstraction that is used
to produce the documentation snippets. A default JMustache-based
implementation is provided. JMustache has been repackaged and embedded
to prevent unwanted conflicts and side-effects.
By default, snippet templates are loaded from the classpath in the
org.springframework.restdocs.templates package. Default snippet
templates are provided for all of the snippets that can be generated.
Each of these templates is named after the snippet that it will
produce – the snippet {name}.adoc is produced by the snippet template
default-{name}.snippet. A snippet named {name}.snippet, if present,
will be used in preference to the default snippet, thereby allowing
the default snippets to be overriden.
