Replace children prop with <slot> tag #104
Description
The web component spec adds a <slot> tag, for handling both default and named slots.
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Slot
https://developer.mozilla.org/en-US/docs/Web/Web_Components/Using_templates_and_slots#Creating_a_new_%3Celement-details%3E_element_from_the_%3Ctemplate%3E
Using <slot> would better align the library with the web component spec, and may allow the library to avoid issues like #101.