Add more demos showing framework integration

[ciampo]

• angular
• vue
• preact
• react
• polymer
• ...

[pimdewit]

@ciampo Would you like to use something like https://codesandbox.io/ for this? or do you prefer to keep it on GitHub?

[ciampo]

The main advantage of using Codesandbox is that you can see/change the code in an interactive way (and it's where the cool kids host their demos as well.)

Hosting them withing the same repo gave me more control over working and iterating with the demos. I'd say, let's keep this issue for actually adding more demos. I've opened issue #49 regarding moving the demos to codesandbox