Rename plugin.go to provider.go in accordance with #12

provider.go

@@ -0,0 +1,26 @@
+package step
+
+import "html/template"
+
+// FuncProvider provides a template.FuncMap.
+type FuncProvider interface {
+	// FuncMap provides a template.FuncMap. It may return nil, in which case
+	// its result is simply not considered.
+	FuncMap () template.FuncMap
+}
+
+// FuncProviderFor is an object that provides a template.FuncMap for a specific
+// document.
+type FuncProviderFor interface {
+	FuncProvider
+	// FuncMap provides a template.FuncMap, given a particular document. It
+	// may return nil, in which case its result is simply not considered.
+	FuncMapFor (*Document) template.FuncMap
+}
+
+// Configurable is an object that can be configured according to metadata.
+type Configurable interface {
+	// Configure uses config to configure the object. It must not modify
+	// config. Keys are namespaced with a '.' and are written in kebab case.
+	Configure (config Meta) error
+}