Tries to fix loading amber from deeply nested pages.

The default is to guess:
  - if amber path is 'xxx/bower_components/yyy',
    use 'xxx/bower_components'
  - otherwise, use 'amber_path/bower_components'

In cases when this is not enough (it should for most cases),
you can specify libs dir in data-libs attribute
of amber.js's <script> tag.