Added keepImport feature + load()/transform() hooks

• updated documentation
• updated dependencies
• fs-extra dependency replaced with mkdirp dependency
• new option keepImport helps to bundle library assets in the way they could be picked up by another bundler (webpack or rollup) and process on their own
• use both load() and transform() hooks to bundle assets, do not run transform() if asset was already transformed or processed using load() hook
• 100% unit test coverage

Regular bundling and with keepImport was verified on 2 different projects.

This release has a lot of new positive changes, however, it still should be backward compatible.