Add Crustache::Engine class for typical using

src/crustache/engine.cr

@@ -0,0 +1,38 @@
+module Crustache
+  class Engine
+    def initialize(@basedir : String, @cache = false)
+      self.initialize ViewLoader.new @basedir, @cache
+    end
+
+    def initialize(@fs : FileSystem); end
+
+    # It renders a template loaded from `filename` with `model`
+    # and it returns rendered string.
+    # If `filename` is not found, it returns `nil`, but it dosen't raise an error.
+    def render(filename : String, model)
+      @fs.load(filename).try{|tmpl| self.render tmpl, model}
+    end
+
+    def render(filename : String, model, output : IO)
+      @fs.load(filename).try{|tmpl| self.render tmpl, model, output}
+    end
+
+    # It is a strict version `Engine#render`.
+    # If `filename` is not found, it raise an error.
+    def render!(filename : String, model)
+      @fs.load!(filename).try{|tmpl| self.render tmpl, model}
+    end
+
+    def render!(filename : String, model, output : IO)
+      @fs.load!(filename).try{|tmpl| self.render tmpl, model, output}
+    end
+
+    def render(tmpl : Template, model)
+      Crustache.render tmpl, model, @fs
+    end
+
+    def render(tmpl : Template, model, output : IO)
+      Crustache.render tmpl, model, @fs, io
+    end
+  end
+end