Provide an option called plugins to process ast in vue-template-compiler (close vuejs#5703)

Author: z-ZYS-s

flow/compiler.js

@@ -21,6 +21,8 @@ declare type CompilerOptions = {
 
   // runtime user-configurable
   delimiters?: [string, string]; // template delimiters
+
+  plugins?: Array<Function>; // customizing plugins to process ast
 };
 
 declare type CompiledResult = {

src/compiler/index.js

@@ -14,6 +14,9 @@ export const createCompiler = createCompilerCreator(function baseCompile (
 ): CompiledResult {
   const ast = parse(template.trim(), options)
   optimize(ast, options)
+  if (options.plugins) {
+    options.plugins.forEach(plugin => plugin(ast, options))
+  }
   const code = generate(ast, options)
   return {
     ast,