Exported source factory

[Venipa]

#77

[odahcam]

Not sure if this is the best way to expose the class, but it is a very interesting idea.

[PowerKiKi]

I don't think the factory should be exposed as is, because of async sources. An async source must be handled differently and you cannot just use getAvatar().

In order to expose the factory, we should modify our Source interface, so that it can always be handled in the same way, something like:

-  getAvatar(size: number): string | null;
+  getAvatar(size: number): Observable<string | null>;

This would also probably simplify our own code...

[odahcam]

The motivation for this is to have the ability to generate avatar URLs trough script instead of HTML, so why not to export the sources instead of the factory? In that way we gotta do less work (which means less risk), users will be able to have the same ability and we won't have to deal with Source interface changes for now, that would speed up acceptance.

[PowerKiKi]

I wouldn't export source implementations, because that would open up a huge new API surface, and that would prevent us from breaking it (as we just did in the several past PRs).

Actually I would suggest to either:

• do nothing at all and let the user re-implement whatever he needs (it's mostly one liners)
• create and export a new Pipe while keeping our implementation details hidden, if it really is needed

[odahcam]

I don't mind about a new API surface, we can let users be responsible for how they use it. We could also separate the URL methods in pure functions and export just them. The Pipe idea seems nice too, but seems to be the harder to implement.