This is an initial implementation of a parser for the WOFF2 font
packaging format.

It is incomplete; further work will come later. The scope for this
milestone was to be able to parse .woff2 files for the needs of the
github.com/ConradIrwin/font/sfnt package.

At this time, the API is very low level and maps directly to the binary
format of the file, as described in its specification. This API is in
early development and is expected to change as further progress is made.

It successfully parses some Go font family .woff2 files that were
generated using the https://github.com/google/woff2 encoder
from the Go font source .ttf files located at
https://go.googlesource.com/image/+/master/font/gofont/ttfs/.

Add basic test coverage.

Helps https://github.com/ConradIrwin/font/issues/1.

Create and describe woff2 package.

Next commit will add an initial implementation.