feat(popover): allow passing custom data to show() method for popovers

[kahboom]

Bug description or feature request:

There is a related issue open for passing custom content to the hide method for the ModalService (#2330). Although popovers do not currently have a service (maybe they should?), we should still be able to pass custom content to the show method of the PopoverDirective, similarly to how ng-bootstrap does: https://ng-bootstrap.github.io/#/components/popover/api

A use case for this is facility in writing plugins for third-party apps, such as ngx-tour. I think there is a way around this by using anonymous arrow functions, but have not tested it yet. Either way, it would make life a lot easier if this were possible.

Versions of ngx-bootstrap, Angular, and Bootstrap:

ngx-bootstrap: 1.9.1

Angular: 4.4.2

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

Angular-CLI: 1.2.x

[kahboom]

In case it helps anyone, my workaround in the meantime was to set it like this, using popoverContext:

this.popover = this.template; // Some popover template.
this.popoverContext = { data }; // Some content being passed to the popover
this.popoverTitle = this.title; // Some title for the popover

[valorkin]

Services is a planned feature, context too