are evaluated for each snapshot, thereby making sure that each
keep-* is able to retain its most recent snapshot. Thereby insuring
that weeklies keep Sundays around and monthlies keep the last day of
the month around.
Added testcase to make sure when multiple --keep-tags are given,
ALL of them need to match.
* convert documentation to reStructuredText
* switch from mkdocs to sphinx
* rewrite README.md (now .rst) to include less information and be more
attractive
