Allow for a customizable fallback mapper

[nanuxbe]

Hello,

I'm not sure whether you're accepting pull requests or not. If you're not, feel free to close!

My thinking here is to be able to set a fallback_mapper in the settings that defaults to dj_angles.mappers.django.map_include.

What one would be able to able to accomplish by switching the fallback mapper is to use their favorite component library as fallback instead of {% include %}.

So for example, assuming django bird, with the appropriate mapper:
<dj-partial /> would be mapped to {% bird partial %}{% endbird %} instead of {% include 'partial.html' %}

I'm opening this as draft, as if that's something you'd be interested in, documentation would be needed