Pass "from" option to PostCSS

This results in better syntax error messages.

README.md

@@ -31,10 +31,14 @@ const filterType = /^text\/css$/
 const html = readFileSync('./index.html', 'utf8')
 
 posthtml([ postcss(postcssPlugins, postcssOptions, filterType) ])
-    .process(html)
+    .process(html, { from: `${__dirname}/index.html` })
     .then((result) => console.log(result.html))
 ```
 
+If you don't pass arguments to `posthtml-postcss`, it will use your project's PostCSS configuration (see [`postcss-load-config`](https://www.npmjs.com/package/postcss-load-config)).
+
+Notice that we're setting the option `from` when calling `process`. `posthtml-postcss` forwards this to PostCSS, which is useful for syntax error messages. (`postcss-cli` and `gulp-posthtml` are setting `from` automatically.)
+
 <h2 align="center">Example</h2>
 
 ```js

index.js

@@ -1,6 +1,17 @@
 var postcss = require('postcss')
+var postcssrc = require('postcss-load-config')
 
 module.exports = function (plugins, options, filterType) {
+  if (Array.from(arguments).length === 0) {
+    try {
+      var rc = postcssrc.sync()
+      plugins = rc.plugins
+      options = rc.options
+    } catch (err) {
+      // noop
+    }
+  }
+
   plugins = [].concat(plugins).filter(plugin => typeof plugin === 'function')
   options = options || {}
 
@@ -24,7 +35,8 @@ module.exports = function (plugins, options, filterType) {
 
         if (meetsFilter) {
           var styles = [].concat(node.content).join('')
-          promise = css.process(styles, options)
+          var from = options.from || tree.options.from
+          promise = css.process(styles, Object.assign({}, options, { from: from }))
             .then(function (result) {
               node.content = [result.css]
             })

package.json

@@ -3,7 +3,9 @@
   "version": "0.3.0",
   "description": "PostHTML-PostCSS plugin. Use PostCSS in HTML document.",
   "main": "index.js",
-  "files": ["index.js"],
+  "files": [
+    "index.js"
+  ],
   "scripts": {
     "test": "npm run lint && npm run coverage",
     "lint": "eslint {test/*,*}.js",
@@ -50,7 +52,8 @@
     "posthtml": "^0.12.0"
   },
   "dependencies": {
-    "postcss": "^7.0.18"
+    "postcss": "^7.0.18",
+    "postcss-load-config": "^2.1.2"
   },
   "eslintConfig": {
     "env": {

test/test.js

@@ -126,4 +126,8 @@ describe('use postcss', function () {
 
     test(html, expected, {}, null, [plugin], done)
   })
+
+  it('doesn\'t crash when no PostCSS configuration is found', function () {
+    expect(function () { posthtml([css()]) }).to.not.throw(Error)
+  })
 })