Feature/support srcset

[brybrophy]

@boygirl @ryan-roemer This PR adds support for srcsets, and satisfies #21 .

I added support in the component, added a srcset to the demo, and added two more test cases.

I also revamped the styles in the demo, using the landing image from the formidable website.

[boygirl]

@brybrophy Thanks! I tested locally, and this looks good to me. New demo page is 💯

[brybrophy]

Great! Thank you.