Add tagging support.

Entries can include a (space-separated) list of tags,
separated from the main entry title by `//`.

Reports include also per-tag totals, if any tags
are found.
Note to self: the above two lines of code are not covered by tests.

(I'm going through [the coveralls report](https://coveralls.io/files/293376059) and checking where those new 6 untested lines of code lie.)