Added
=====

- `Face::empty` constructor as a simple way to construct the empty face
- `Feature` struct that wraps `hb_feature_t` and has an easy to use constructor
- `UnicodeBuffer::add_str_item` to allow providing context to the string
  being shaped.
- `UnicodeBuffer::preallocate`
- Reexport of `harfbuzz_sys` as `hb` to facilitate use of unwrapped functions

Changed
=======

- removed kerning callbacks from FontFuncs (following the upstream harfbuzz
  change). This also enabled updating to harfbuzz-sys 0.3.
- updated to use Rust 2018
- Further improved docs