Support for one-to-many or many-to-many links

[samber]

I added to my Angular2 fork an option to use a class, instead of an id for selecting from/to elements.

<ngx-domarrow from="#single" to=".multiple" [tail]="true"
        color="#0000FF"></ngx-domarrow>

(green arrows)

samber/ngx-domarrow@967c202

It won't be easy to create a PR from my repo, since I made a lot of breaking changes. I will write a PR ASAP.

[schaumb]

Thanks! 👍