Whitespace only: improve readability of post generation code.
Use explicit constant to allow more independent test running.
Make the previous and next posts available on individual post pages.

This adds two template variables, "prev_post" and "next_post", to the
individual post templates, which contain the full Post object.

Related to #3.
Mention {next,prev}_post in CHANGELOG.
Touch on post template variables in the README.
Merge pull request #3 from aprescott/prev-next-post

Support getting previous and next posts from a given post