Add possibility to exclude certain parts from the build

[anehx]

This PR adds some options and fixes #12:

'ember-uikit': {
  importUIkitCSS: true, // excludes css from build if false
  importUIkitJS: true, // excludes js from build if false
  importUIkitAssets: true, // excludes assets (not icons) from build if false
  importUIkitIcons: true, // excludes icons from build if false

  useIcons: true, // excludes all icon related stuff if false

  whitelist: [], // components which will be included
  blacklist: [] // components which will be excluded
}

TODO

• Document options
• Exclude uk-icon if useIcons: false

[jayvarner]

The treeshake pattern is very elegant. Documentation seems clear. Nice work!

[czosel]

If you say that the two lists should not be used at the same time, i'd expect an error to be thrown in that case. I think right now the blacklist takes precedence.

[anehx]

That should definitely throw an error!

[anehx]

Or at least a warning

[czosel]

Apart from the nitpick, LGTM!