Change Rust source files to includes

[nathantypanski]

This change creates a _includes directory in the _docs directory and
adds includes for the relevant source files instead of copying them
directly into the markdown posts.

The _includes directory is actually a relative symlink to the patterns
directory, i.e,

docs$ ls -l _includes
total 4
lrwxrwxrwx 1 nathan nathan 14 Jul 12 23:03 patterns -> ../../patterns

meaning it should work on any system that understands symlinks.

This means that changes to the Rust files are automatically reflected in
the rendered pages by Jekyll.

Closes #3.