Add expiration format option

Allow expiration dates on todos, something like `todo(name): message, expires(2014-01-20)` or `expires(May 2010)` - use momen.js for parsing

Review the existing TODO-format parsing entry point and test coverage first; no files or tests are named in the issue. Clarify the accepted expiration syntax, supported date formats, and expected behavior before implementing parsing with moment.js, then add coverage showing that expiration dates are recognized.

Written by the indexing model from the issue text.