Add optional freetype integration

[narodnik]

Added the function hb_ft_font_create_referenced() for creating fonts from freetype faces.

[narodnik]

When building for android target, the compiled harfbuzz is missing freetype support due to the system not having it installed.

Should we make these changes:

1. Link freetype-sys as a git submodule
2. Add these lines to build.rs: .include("freetype-sys/freetype2/include/") and cfg.define("HAVE_FREETYPE", "1");.

?

[narodnik]

so....