Support for linked posts.

Cycle 0.2.2

feat: simplify travis binary build

feat: Multi-stage Docker build

We can cut down on the size of the final image by breaking the build
up into two stages: compilation and execution. Since we don't need the
compilation dependencies to execute cycle we create one image which
has those then create a new image that does not, copy cycle into the
new smaller image, then discard the original.

feat: build mac binary on travis

refactor: remove hand-rolled rfc-822 date formatting for local-time

fix: check if uiop:subdirectories returns nil before processing

feat: wrap up hard-coded file paths and data structures in checks.

fix: make sure site/writing dirs are created.

feat: allow archive to not have any pagination.

- pass in a zero for the paginate value and gen-archive will return
  the full list of posts.