Feature: Allow filtering for multiple leagues

[sgurlt]

Within my custom theme, I would like to be able to get data from the calendar for one, or multiple leagues.
At the Moment this is not possible due to the fact that the handled $this->league variable, always gets casted into an array.

To be able to handle one or multiple values, I would like to add a check if the handed in data is already an array, or not.

This might also be useful for seasons and venues, but is currently outside the scope of my project.