Improve config merge approach

[EvanLovely]

The sass plugin default config has this in it:

const defaultConfig = {
  autoPrefixerBrowsers: [
    'last 2 versions',
    'IE >= 10',
  ],
};

And in my project I use this config:

const config = {
  autoPrefixerBrowsers: [
    'last 2 versions',
  ],
};

Since we're using Lodash's merge (via @theme-tools/core) to merge the two together, we basically get the default config above. Ideally, all of autoPrefixerBrowsers would overwritten when in the user config, so the first instinct is to simply use Object.assign, but then we get into issues concerning nested config like sassdoc:

const defaultConfig = {
  src: [
    'scss/**/*.scss',
  ],
  // more options here...
  sassdoc: {
    enabled: false,
    dest: 'dest/sassdoc',
    verbose: false,
    basePath: '',
    exclude: [],
    theme: 'default',
    sort: [
      'file',
      'group',
      'line>',
    ],
  },
};

Ideally, the user could get set config.sassdoc.enabled to true and then have those other defaults set that are inside the sassdoc object.

[EvanLovely]

If we move to Object.assign, this could be a good disclaimer:

Note: The merge of variables in these files is shallow, if you want to override a single item in a list, you will need to re-define all items in that list.

[sghoweri]

Hmmmm. I might take a peek at this once I get back from my run. Feel like I might have worked through something similar when putting together a couple POC build tasks for the new design system revamp...

[sghoweri]

@EvanLovely something like #12? Is that about right?

[EvanLovely]

Perfect! Thanks!