Add support for custom text escaping (#11)

* Add support for custom text escaping

* swift format

* Remove withDefaultDelimiters

* Update README.md

* Don't pass content type into partial

Sources/HummingbirdMustache/ContentType.swift

@@ -0,0 +1,43 @@
+/// Protocol for content types
+public protocol HBMustacheContentType {
+    /// escape text for this content type eg for HTML replace "<" with "&lt;"
+    func escapeText(_ text: String) -> String
+}
+
+/// Text content type where no character is escaped
+struct HBTextContentType: HBMustacheContentType {
+    func escapeText(_ text: String) -> String {
+        return text
+    }
+}
+
+/// HTML content where text is escaped for HTML output
+struct HBHTMLContentType: HBMustacheContentType {
+    func escapeText(_ text: String) -> String {
+        return text.htmlEscape()
+    }
+}
+
+/// Map of strings to content types.
+///
+/// The string is read from the "CONTENT_TYPE" pragma `{{% CONTENT_TYPE: type}}`. Replace type with
+/// the content type required. The default available types are `TEXT` and `HTML`. You can register your own
+/// with `HBMustacheContentTypes.register`.
+public enum HBMustacheContentTypes {
+    static func get(_ name: String) -> HBMustacheContentType? {
+        return self.types[name]
+    }
+
+    /// Register new content type
+    /// - Parameters:
+    ///   - contentType: Content type
+    ///   - name: String to identify it
+    public static func register(_ contentType: HBMustacheContentType, named name: String) {
+        self.types[name] = contentType
+    }
+
+    static var types: [String: HBMustacheContentType] = [
+        "HTML": HBHTMLContentType(),
+        "TEXT": HBTextContentType(),
+    ]
+}