Add percent units to color stops

[acupoftee]

This PR address issue #13

Percentage units were added to all four gradient properties in codeblock.jsx.

This produces the correct CSS. Here's an example for XPro2:

Full CSS:

background: -webkit-radial-gradient(50% 50%, circle closest-corner, rgba(224, 231, 230, 1) 40%, rgba(43, 42, 161, 0.6));
background: radial-gradient(50% 50%, circle closest-corner, rgba(224, 231, 230, 1) 40%, rgba(43, 42, 161, 0.6));