Add hooks in Javascript layer to enable customizations of calendar

[davefx]

Problem this Pull Request solves

This PR allows the full customization of the calendar component.

How has this been tested

I'm working on a plugin that integrates the Event Calendar with other plugins from the site, and these changes were needed in order to customize the calendar's behaviour.

Checklist

• [x] I have read the documentation relating to systems affected by this pull request, see https://github.com/eventespresso/event-espresso-core/tree/master/docs
• [x] User input is adequately validated and sanitized
• [x] all publicly displayed strings are internationalized (usually using esc_html__(), see https://codex.wordpress.org/I18n_for_WordPress_Developers)
• [x] My code is tested.
• [x] My code follows the Event Espresso code style.
• [ ] My code has proper inline documentation.