Feature/today button + fix for previous navigation button

[EmCousin]

This Pull Request addresses https://github.com/excid3/simple_calendar/issues/271 and https://github.com/excid3/simple_calendar/issues/272

• [x] Added a "today" button in addition to "previous" and "next" (https://github.com/excid3/simple_calendar/issues/271)
• [x] Fixed "previous" button for generic calendars (https://github.com/excid3/simple_calendar/issues/272)
• [x] Added tests to cover these improvements + wrote some overdue tests for week and month calendars
• [x] Updated documentation (README.md)
• [x] Updated changelog (CHANGELOG.md)

[excid3]

cc @cjilbert504

[EmCousin]

@excid3 I'm not entirely sure to understand what's going on with CI :

 You are trying to install in deployment mode after changing
  your Gemfile. Run `bundle install` elsewhere and add the
  updated gemfiles/rails_6_1.gemfile.lock to version control.

This PR didn't touch the Gemfile. Is there anything I'm missing?

[rodrigoargumedo]

Did you make an update to Rails 6.1.x Gemfile?

[EmCousin]

Hi @rodrigoargumedo, not that I know of. I see that the repo has multiple Gemfiles so that CircleCI runs the tests over multiple environments, but this branch didn't touch any of them. Should it?

[zwfisher]

this would be a nice change to go with new tailwind calendars, any word on getting those tests to pass?