Add ability to pass in css class to spinner and bar

[richarddavenport]

Closes #141 Not sure what you'd like to call the inputs, feel free to change them.

[richarddavenport]

I realized that the background was overwriting some styles in my css class, namely background-color. I'm guessing that most people are wanting to style it based on a class, so the loader probably shouldn't set background in that case. Also, my [ngClass] code was old and wrong, updated to work.

[aitboudad]

could you please update readme https://github.com/aitboudad/ngx-loading-bar#customize-ngx-loading-bar

[aitboudad]

I'm not sure if we do really need such inputs 🤔, to use scss variables just target the used classes ngx-spinner and ngx-bar. let me know WDYT?

[moberwasserlechner]

@aitboudad Thx for the hint.

Styling the loading bar with

.ngx-bar {
  background-color: mat-color($accent) !important;
}

works great and I can use Angular Material Theming that way.

It did not work without the !important though as the fallback color is always set to element style.

[moberwasserlechner]

I have to revoke my previous comment. With proper Angluar 12 theming it does not work. This PR would be really great.