Default snippet encoding to UTF-8 and make it configurable

Previously, snippets were written to disk using the JVM’s default
encoding. While this could be controlled by the file.encoding system
property it was not ideal as, if the proper was not always set, it could
lead to platform-dependent output being generated. Furthermore,
Asciidoctor uses UTF-8 encoding by default so there was a risk of the
snippets being generated in a different encoding to the encoding
expected by Asciidoctor.

This commit updates the configuration so that, by default, snippets are
encoded as UTF-8. The RestDocumentationConfigurer API has been enhanced
to allow this default to be configured as part of building the
MockMvc instance. This enhancement as brought with it a more fluent
configuration API that separates configuration that is related to URIs
from configuration that is related to snippets.

Closes gh-67

README.md

@@ -243,24 +243,48 @@ documenting. A custom `ResultHandler` is used to produce individual documentatio
 snippets for its request and its response as well as a snippet that contains both its
 request and its response.
 
-You can configure the scheme, host, and port of any URIs that appear in the
-documentation snippets:
+The first step is to create a `MockMvc` instance using `MockMvcBuilders`, configuring
+it by applying a `RestDocumentationConfigurer` that can be obtained from the static
+`RestDocumentation.documentationConfiguration` method:
 
 ```java
 @Before
 public void setUp() {
 	this.mockMvc = MockMvcBuilders
 			.webAppContextSetup(this.context)
-			.apply(new RestDocumentationConfigurer()
-					.withScheme("https")
-					.withHost("localhost")
-					.withPort(8443))
+			.apply(documentationConfiguration())
 			.build();
 }
 ```
 
-The default values are `http`, `localhost`, and `8080`. You can omit the above
-configuration if these defaults meet your needs.
+This will apply the default REST documentation configuration:
+
+| Setting          | Default value
+| ---------------- | -------------
+| Scheme           | http
+| Host             | localhost
+| Port             | 8080
+| Context path     | Empty string
+| Snippet encoding | UTF-8
+
+One or more of these settings can be overridden:
+
+```java
+@Before
+public void setUp() {
+	this.mockMvc = MockMvcBuilders
+			.webAppContextSetup(this.context)
+			.apply(documentationConfiguration()
+					.uris()
+							.withScheme("https")
+							.withHost("api.example.com")
+							.withPort(443)
+							.withContextPath("/v3")
+					.and().snippets()
+							.withEncoding("ISO-8859-1"))
+			.build();
+}
+```
 
 To document a MockMvc call, you use MockMvc's `andDo` method, passing it a
 `RestDocumentationResultHandler` that can be easily obtained from