Add options for imports and plugins

[craigbeck]

Move dependency on stylus to peerDependency and provide options to provide imports paths for stylesheet resolution from node_modules, and plugins for specifying stylus plugins to use (like nib)

Current patched version 0.1.3 still has dependency on nib as plugin, conflicting with v0.2.0 and later versions of this plugin that remove nib as dependency. Nib, as a plugin, may not be the right choice, or may conflict with other plugins desired, so removing this dependency and providing for option to indicate plugins to be used is provided:

Usage:

app.serverUse(module, '@derbyjs/derby-stylus', {
  imports: ['branded-styles'],            // adds `branded-styles` package from `node_modules` to `paths`
  plugins: ['nib'],                       // `nib` is a dependency, specified as string
});

and stylesheets can require without relative pathing to node_modules

// for index.styl
@require 'branded-styles` 
@require 'branded-styles/icons`

// for `icons.styl`
@requrie 'branded-styles/icons'

instead of things like ../../node_modules/branded-styles

[ericyhwang]

From the PR description, it's not clear to be what the practical benefit of this would be. Can you add an example of the effects of using the new options?

[ericyhwang]

Would be better to use path.dirname() here for Windows compatibility

[ericyhwang]

More of a stylistic thing, I think forEach makes for easier to read code than reduce

[ericyhwang]

Using default params and spread operator means this is no longer compatible with ES5, so reminder that the new minimum of ES6 (ES2015) should be called out in the release notes

[ericyhwang]

It looks like opts is intended to be a base set of options to be passed directly to the Stylus compiler?

If that's the case, I'd prefer them to be passed as an stylusOptions property for clarity, instead of intermingled with the Derby plugin options.

Can also name the local var to emphasize that it's a base set of Stylus options, since it can be overridden later by individual loadStylesSync calls (also see comment further below)

var baseStylusOptions = options.stylusOptions;
...
function stylusCompiler(file, filename, options) {
  var options = { _imports: [], ...baseStylusOptions, ...options };
  ...

[ericyhwang]

The options argument here was passed in a specific loadStylesSync call:
https://github.com/derbyjs/derby/blob/bcdbeaccb0a0fc0293022ce3574d3f414bcf4f42/src/files.ts#L59-L76

So the more specific call's options should take precedence over the base options provided to the plugin as a whole.