Add badge/shield image API

[styfle]

Users might want to show that their package is relatively small by displaying a badge on their GitHub README.md to show the world.

[styfle]

Here's an idea, use the color to show how large it is, green being small and red being large.

[styfle]

it works!

Here is what the colors look like:

package	install size
copee
node-fetch
lodash
webpack
webpack-cli
electron