feat(options): add optional Options arguments to New() function (#14)

Enables simpler creation of custom *Golden instances, as you can just pass in the custom values to New(), rather than modifying fields after increating the Golden instance.

options.go

@@ -0,0 +1,43 @@
+package golden
+
+import (
+	"os"
+)
+
+// Option is a function that modifies a Golden instance.
+type Option func(*Golden)
+
+// WithDirMode sets the directory mode for a Golden instance.
+func WithDirMode(mode os.FileMode) Option {
+	return func(g *Golden) {
+		g.DirMode = mode
+	}
+}
+
+// WithFileMode sets the file mode for a Golden instance.
+func WithFileMode(mode os.FileMode) Option {
+	return func(g *Golden) {
+		g.FileMode = mode
+	}
+}
+
+// WithSuffix sets the file suffix for a Golden instance.
+func WithSuffix(suffix string) Option {
+	return func(g *Golden) {
+		g.Suffix = suffix
+	}
+}
+
+// WithDirname sets the directory name for a Golden instance.
+func WithDirname(dirname string) Option {
+	return func(g *Golden) {
+		g.Dirname = dirname
+	}
+}
+
+// WithUpdateFunc sets the update function for a Golden instance.
+func WithUpdateFunc(updateFunc UpdateFunc) Option {
+	return func(g *Golden) {
+		g.UpdateFunc = updateFunc
+	}
+}