Feat/expanded shortcuts

[eric-norcross]

This PR adds support for the following custom-shortcuts:

{ key: 'nextWeek', label: 'Next week', value: '+isoWeek' },
{ key: 'nextMonth', label: 'Next month', value: '+month' },
{ key: 'nextYear', label: 'Next year', value: '+year' }

Above functionality has been reflected in the README as well as the demo examples page.