Add template library loading from FileSystem

Sources/HummingbirdMustache/Library.swift

@@ -1,9 +1,14 @@
 
 public class HBMustacheLibrary {
-    init() {
+    public init() {
         self.templates = [:]
     }
     
+    public init(directory: String) {
+        self.templates = [:]
+        self.loadTemplates(from: directory)
+    }
+    
     public func register(_ template: HBMustacheTemplate, named name: String) {
         templates[name] = template
     }