Add monthChanged Event

[alexander-merz]

Hi there 👋

For my personal project I am in desperate need of an indication for month and year changes.

• Fire a monthChanged event whenever month or year has changed.
• Emit { month, year } as event details

[Sqrrl]

Thanks. Much appreciated. It's published as v0.3.0.

I made some minor changes:

• renamed the event to changeMonth to be consistent with other present tense event names
• made the month value run from 1 to 12 (instead of 0 to 11) for consistency reasons
• added the event description to the docs page