Replace grunt-postcss with postcss-cli (twbs#20140)

Refs twbs#19990
Continues the degruntification process.

Also removes mq4-hover-shim for now,
since it doesn't yet implement the standard PostCSS plugin interface.

Gruntfile.js

@@ -19,9 +19,6 @@ module.exports = function (grunt) {
   var fs = require('fs');
   var path = require('path');
   var isTravis = require('is-travis');
-  var mq4HoverShim = require('mq4-hover-shim');
-  var autoprefixerSettings = require('./grunt/autoprefixer-settings.js');
-  var autoprefixer = require('autoprefixer')(autoprefixerSettings);
 
   var configBridge = grunt.file.readJSON('./grunt/configBridge.json', { encoding: 'utf8' });
 
@@ -166,39 +163,6 @@ module.exports = function (grunt) {
       }
     },
 
-    postcss: {
-      core: {
-        options: {
-          map: true,
-          processors: [
-            mq4HoverShim.postprocessorFor({ hoverSelectorPrefix: '.bs-true-hover ' }),
-            require('postcss-flexbugs-fixes')(),
-            autoprefixer
-          ]
-        },
-        src: 'dist/css/*.css'
-      },
-      docs: {
-        options: {
-          processors: [
-            autoprefixer
-          ]
-        },
-        src: 'docs/assets/css/docs.min.css'
-      },
-      examples: {
-        options: {
-          processors: [
-            autoprefixer
-          ]
-        },
-        expand: true,
-        cwd: 'docs/examples/',
-        src: ['**/*.css'],
-        dest: 'docs/examples/'
-      }
-    },
-
     cssmin: {
       options: {
         // TODO: disable `zeroUnits` optimization once clean-css 3.2 is released
@@ -308,6 +272,12 @@ module.exports = function (grunt) {
     },
 
     exec: {
+      postcss: {
+        command: 'npm run postcss'
+      },
+      'postcss-docs': {
+        command: 'npm run postcss-docs'
+      }
     },
 
     buildcontrol: {
@@ -403,7 +373,7 @@ module.exports = function (grunt) {
   // grunt.registerTask('sass-compile', ['sass:core', 'sass:extras', 'sass:docs']);
   grunt.registerTask('sass-compile', ['sass:core', 'sass:docs']);
 
-  grunt.registerTask('dist-css', ['sass-compile', 'postcss:core', 'cssmin:core', 'cssmin:docs']);
+  grunt.registerTask('dist-css', ['sass-compile', 'exec:postcss', 'cssmin:core', 'cssmin:docs']);
 
   // Full distribution task.
   grunt.registerTask('dist', ['clean:dist', 'dist-css', 'dist-js']);
@@ -412,7 +382,7 @@ module.exports = function (grunt) {
   grunt.registerTask('default', ['clean:dist', 'test']);
 
   // Docs task.
-  grunt.registerTask('docs-css', ['postcss:docs', 'postcss:examples', 'cssmin:docs']);
+  grunt.registerTask('docs-css', ['cssmin:docs', 'exec:postcss-docs']);
   grunt.registerTask('lint-docs-css', ['scsslint:docs']);
   grunt.registerTask('docs-js', ['uglify:docsJs']);
   grunt.registerTask('docs', ['lint-docs-css', 'docs-css', 'docs-js', 'clean:docs', 'copy:docs']);

docs/getting-started/build-tools.md

@@ -46,7 +46,7 @@ For example, run `TWBS_SASS=sass grunt` to test and build Bootstrap with Ruby Sa
 
 Bootstrap uses [Autoprefixer][autoprefixer] (included in our Gruntfile and build process) to automatically add vendor prefixes to some CSS properties at build time. Doing so saves us time and code by allowing us to write key parts of our CSS a single time while eliminating the need for vendor mixins like those found in v3.
 
-We maintain the list of browsers supported through Autoprefixer in a separate file within our GitHub repository. See [`/grunt/autoprefixer-settings.json`](https://github.com/twbs/bootstrap/blob/master/grunt/autoprefixer-settings.js) for details.
+We maintain the list of browsers supported through Autoprefixer in a separate file within our GitHub repository. See [`/grunt/postcss.js`](https://github.com/twbs/bootstrap/blob/master/grunt/postcss.js) for details.
 
 ## Local documentation