Is your feature request related to a problem? Please describe...

I am building a custom datepicker based on the calendar component which requires that when tabbing through inputs, focus not be shifted to the calendar but rather to the next input.

Describe the solution you'd like

Expose a new parameter for b-calendar, tabindex with a default value of 0

Describe alternatives you've considered

Manually updating tabindex values rendered by the component on mount, jquery style. Unpleasant and anti-pattern.

Additional context

As added value, this feature can solve problems for people with complex tab layouts ( layouts that call for more than just -1 and 0 values )