Support title-less feeds

[jaredcwhite]

For better support of microblogging contexts where a title key may not be present in the front matter (or an empty string depending on how a CMS might handle things), we should simply omit the title tag in the feed output for the entry.

[jaredcwhite]

Update: this actually is dependent on #5 as the Atom format doesn't actually allow feed items without titles. RSS 2.0 allows either a title tag or a description tag. Guess we'll be switching!