Add highlighted and disabled classes

[mst101]

Hi @MrWook, Happy New (calendar) Year!

In an effort to simplify the code, I made the following changes for your consideration:

• Created two new classes (DisabledDate.js and HighlightedDate.js) in utils to determine whether a particular cell should be disabled and/or highlighted.

• Both classes expose a config property which easily allows us to determine whether a disabled/highlighted configuration exists and if so, whether it has from, to, specificDates, daysOfMonth, daysOfWeek, customPredictor etc. properties. These boolean properties clean up quite a lot of code in other places. e.g. if (!this.disabledDates || !this.disabledDates.to) becomes if (!this.disabledConfig.has.to).

• The config properties also contain a to and from object for easy access to e.g. this.disabledConfig.to.year as opposed to this.utils.getFullYear(this.disabledDates.to)

• Given that highlighted dates are uniquely relevant to PickerDay.vue, I added a highlightedConfig computed property ONLY for that component and a disabledConfig computed property to pickerMixin as it's used in all 3 pickers.

• To improve readability, I added a pageMonth computed property to PickerDay and a pageYear computed property to pickerMixin. This means we can write e.g. this.pageMonth instead of this.utils.getMonth(this.pageDate).

• Added a test:debug script to package.json to help with debugging tests via Chrome dev tools.

• Corrected some December dates in highlightedDates.spec.js e.g. new Date(2016, 12, 8) -> new Date(2016, 11, 8).

• Added a test to disabledMonths.spec.js to ensure coverage remains at 100%. This checks that a date's month is disabled when the to year is less than the year of the date in question, thereby covering line 112 of DisabledDate.js.

Hope this makes sense? Let me know if you have any queries.

[mst101]

I realised the setters I had written on these classes were pointless, so I removed them. However, if you think we should do some validation on the disabled-dates and highlighted props, let me know and I'll add that.

[MrWook]

On the first glance it looks pretty neat to me, i will take a deeper look tomorrow and test it out 👍
Thanks for the improvements 👍

[mst101]

@MrWook - thanks for your improvements which I have now implemented. Cheers! 😄