Allow to configure custom element tagname

[brunob15]

By default the custom element generated has a ngx-element tagname.
Enable to pass a elementTagName to the configuration passed to NgxElementModule.forRoot() in order to let the user customize it.

[michaelkrog]

This would be awesome. If a proxy could be registered as a custom element for each of the selectors in the configuration (with the selector as the tag name) - and if the PR related to @Output fell in place - then it would be almost perfect. 👍

[brunob15]

Hi @michaelkrog, feel free to open a PR if you want 🙂