Add library and support for partials

Sources/HummingbirdMustache/Library.swift

@@ -0,0 +1,17 @@
+
+public class HBMustacheLibrary {
+    init() {
+        self.templates = [:]
+    }
+    
+    public func register(_ template: HBMustacheTemplate, named name: String) {
+        templates[name] = template
+    }
+    
+    public func render(_ object: Any, withTemplateNamed name: String) -> String? {
+        guard let template = templates[name] else { return nil }
+        return template.render(object, library: self)
+    }
+    
+    var templates: [String: HBMustacheTemplate]
+}