World calendars open items

[alexcjohnson]

A couple of things that came up in #1220 that would be nice to add:

• support dtick='Y<n>' (for years) like we do 'M<n>' for months. For the regular Gregorian calendar this doesn't matter, as a year is always 12 months, but for other calendars we currently hack in year ticks by assuming that if n is a multiple of 12 the user really meant years because that's what they do for the Gregorian calendar. We should just make explicit year ticks. Perhaps it would be nice to make other units too, like day (if we ever put back in timezones the right way, we will need this to handle 23 and 25-hour days), hour, minute (shoot, that should have gotten the capital M for consistency. oops), second, though the latter ones are for convenience only. See World calendars #1220 (comment)

• support year to date and month to date range selectors with non-Gregorian calendars. See World calendars #1220 (comment)