Allow custom attrToProp function

[Siilwyn]

Would it be a good idea to allow attrToProp to accept a function? That way as a user I can transform the needed attributes. I'm mainly asking this because the hyperscript-attribute-to-property dependency is missing basic attributes like srcset & seems unmaintained. Secondly it does not include SVG attributes like stroke-linecap which in my case are used directly in the templating.

[feross]

Just wanted to note that hyperscript-attribute-to-property is maintained.