feat(index): tree fromString/toString, close #242

lib/index.js

@@ -51,6 +51,50 @@ function PostHTML (plugins) {
    */
   this.messages = []
 
+  /**
+   * Tree method parsing string inside plugins.
+   *
+   * @memberof tree
+   * @type {Function} fromString
+   *
+   * @example
+   * ```js
+   * export default function plugin (options = {}) {
+   *   return function (tree) {
+   *      tree.match({ tag: 'include' }, function(node) {
+   *          node.content = tree.fromString(fs.readFileSync(node.attr.src))
+   *      })
+   *
+   *      return tree
+   *   }
+   * }
+   * ```
+   */
+  this.fromString = parser
+
+  /**
+   * Tree method rendering tree to string inside plugins.
+   *
+   * @memberof tree
+   * @type {Function} toString
+   *
+   * @example
+   * ```js
+   * export default function plugin (options = {}) {
+   *   return function (tree) {
+   *      function (tree) {
+   *         var outherTree = ['\n', {tag: 'div', content: ['1']}, '\n\t', {tag: 'div', content: ['2']}, '\n'];
+   *         var htmlWitchoutSpaceless = tree.toString(outherTree).replace(/[\n|\t]/g, '');
+   *         return tree.fromString(htmlWitchoutSpaceless)
+   *      }
+   *
+   *      return tree
+   *   }
+   * }
+   * ```
+   */
+  this.toString = render
+
   // extend api methods
   Api.call(this)
 }
@@ -102,8 +146,8 @@ PostHTML.prototype.process = function (tree, options) {
    */
   options = this.options = options || {}
 
-  if (options.parser) parser = options.parser
-  if (options.render) render = options.render
+  if (options.parser) parser = this.fromString = options.parser
+  if (options.render) render = this.toString = options.render
 
   tree = options.skipParse
     ? tree || []