Bug - enable option should support function

[Boxuan Li]

According to flatpickr documentation and code, enable can be an array of functions.

https://github.com/flatpickr/flatpickr/blob/7bd701c656d5946f10328f9ae874c0d80bd46f1b/src/types/options.ts#L9

However, ng2-flatpickr defines enable as an array of string/date.

ng2-flatpickr/src/flatpickr-options.interface.d.ts

Line 12 in 9b519cc

 enable?: Array<string | Date>; 

[Boxuan Li]

For now, I use

// @ts-ignore

to ignore typescript error as a workaround.