Allow user to save an SVG

[rupl]

Public demonstrations are fun, but opening devtools to save a WIP as SVG is not 😁

Having a button that saves the SVG quickly would be very rad. It also opens the door to possibilities such as #18, hosting artwork that people create.

• [x] Add Modernizr test for blob downloads
• [x] Create button that saves WIP canvas as SVG
• [x] Randomize timestamp filename so we can quickly save without overwriting. Could just sort a directory by creation date later on to organize them.
• [x] Find a home for button, style it
• [x] Suppress button on mobile

[rupl]

👍 solid!