[feature] Support relative weeks (#5461)

[feature] Support relative weeks (#5461)

This patch adds support for relative time in the form '%d weeks'. It is
by default disabled. To enable it set relativeTimeThreshold for 'w' to
a number (like 4), and optionally lower the 'd' threshold (like 7), so
anything >= 7 days is considered a week, and 4 weeks are considered
a month.