Make date formats more configurable#42

Stemming from https://github.com/philgyford/django-hines/issues/205 Although we [added](https://github.com/philgyford/django-spectator/issues/27) the `SPECTATOR_DATE_FORMAT` setting, this isn't used when we're generating a span of dates for readings. e.g. "1 February 2017 to 2018", "1-6 February 2017" or "February to March 2018". So I think we need a bunch of settings to be used by `spectator.reading.templatetags.spectator_reading.reading_dates()`. Initial guesses, with defaults: ```python SPECTATOR_DATE_FORMAT = "%-d %B %Y" # "3 September 2017", already exists SPECTATOR_DATE_FORMAT_MONTH_YEAR = "%B %Y" # "September 2017" SPECTATOR_DATE_FORMAT_DAY_MONTH = "%-d %B" # "3 September" SPECTATOR_DATE_FORMAT_YEAR = "%Y" # "2017" SPECTATOR_DATE_FORMAT_MONTH = "%B" # "September" SPECTATOR_DATE_FORMAT_DAY = "%-d" # "3" SPECTATOR_PERIOD_FORMAT_SHORT = "{}–{}" # Like "3–5 September 2017" SPECTATOR_PERIOD_FORMAT_LONG = "{} to {}" # Like "3 September 2017 to 3 March 2018" ``` **But**... while I want to be able to make all day-specific periods for Hines like "2019-10-28", doing the above alone wouldn't let us generate links for each date to that day's page.