Add support for adding features

<a href="https://github.com/deepakjois"><img src="https://avatars1.githubusercontent.com/u/5342?v=3" align="left" width="96" height="96" hspace="10"></img></a> **Issue by [deepakjois](https://github.com/deepakjois)**
_Friday May 19, 2017 at 10:50 GMT_
_Originally opened as https://github.com/deepakjois/luatex-harfbuzz/issues/3_

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