support postcss

[olets]

To test, add vigetlabs/blendid#postcss as a project dependency, and confirm that stylesheets are processed correctly. Tested with CSS (source files .css, stylesheets.sass false) and Sass (source files .scss, stylesheets true), and custom PostCSS config as below)

• Adds PostCSS support
• Autoprefixer replaces gulp-autoprefixer
• cssnano replaces gulp-cssnano

Users can configure plugins and options in task-config.js's stylesheets.postcss. See gulp-postcss for more info.

Basic usage is unchanged. Source stylesheets will be preprocessed with Sass unless stylesheets.sass is false. You can still call out Sass explicitly if you like:

// in task-config.js
stylesheets: true

A task-config with custom PostCSS will look like this

// task-config.js
// must also add the dependencies (`(npm i|yarn add) some-plugin some-option`)

var somePlugin = require('some-plugin')
var someOption = require('some-option')

var postCssPlugins = [somePlugin()]
var postCssOptions = {someOption}

module.exports = {
    // ...
    stylesheets: {
        // sass: true is implied
        postcss: {
           plugins: postCssPlugins,
           options: postCssOptions
        }
    }
    // ...
}

Autoprefixer and cssnano are injected into the PostCSS plugins list, and do not need to be specified. However custom Autoprefixer and/or cssnano configs are respected if provided. That looks like this:

// task-config.js
// must also add the autoprefixer dependency (`(npm i|yarn add) autoprefixer`)

var autoprefixer = require('autoprefixer')

var postCssPlugins = [
    autoprefixer({
        grid: "autoplace"
    })
]

module.exports = {
    // ...
    stylesheets: {
        // sass: true is implied
        postcss: {
           plugins: postCssPlugins
        }
    }
    // ...
}

[olets]

This is merged.