feat(date/time-picker): add possibility to set offsets

[juicyarts]

Bug description or feature request:

I searched for this kind of issues but couldn't find anything that is comparable to our use-case and proposal.

use case:

We use the date and time-pickers of ngx-bootstrap in an application that needs to show all date and time values in a specific timezone. Given some companies that a user has access to, the date and time values shown in the application should all refer to the timezone the selected company is operating in no matter where the user/client is located.

I started implementing this in a fork since we definitely need this but if you also see this as a useful feature it would be nice to implement it into ngx-bootstrap.

proposal

A new property called offset is made available

...
@Input() offset: number; // time offset in minutes, can be negative
...

<timepicker [(ngModel)]="mytime" [offset]="offset"></timepicker>
<datepicker [(ngModel)]="mytime" [offset]="offset"></datepicker>

This adds or subtracts given offset to the "displayed" values and also takes them into account when working with min and max properties. The date object itself is not being modified since this is all about the display format.

Versions of ngx-bootstrap, Angular, and Bootstrap:

ngx-bootstrap: 3.1.0

Angular: 6.1.6

Bootstrap: 4

Build system: Angular CLI, System.js, webpack, starter seed:

CLI