feat(loading): support for async and boolean with [until] input. (closes #528)

[emoralesb05]

#528

introducing a new syntax to use the tdLoading directive which lets the users leverage the async angular pipe and assign a reference from the result to use within the context.

also leveraging this, we can use booleans to show and hide anonymous loading directives.

What's included?

• new [tdLoadingUntil] input to support both async and boolean cases.
• new syntax to use with tdLoading for until usage:

// template syntax
<ng-template let-items tdLoading [tdLoadingUntil]="listObservable | async">
   {{items}}

OR

// star syntax
<div *tdLoading="let items until listObservable | async; type: circle; color: accent">
   {{items}}

• usage for boolean case:

<ng-template tdLoading [tdLoadingUntil]="!loading" tdLoadingStrategy="overlay">

• Updated unit tests and documentation

Test Steps

• ng serve
• Go to http://localhost:4200/#/components/loading
• Play with demos and see how both cases work

General Tests for Every PR

• ng serve --aot still works.
• npm run lint passes.
• npm test passes and code coverage is not lower.
• npm run build still works.

Screenshots or link to CodePen/Plunker/JSfiddle