### extendHex

Extends a 3-digit color code to a 6-digit color code.

Use `Array.map()`, `split()` and `Array.join()` to join the mapped array for converting a 3-digit RGB notated hexadecimal color-code to the 6-digit form.
`String.slice()` is used to remove `#` from string start since it's added once.
```js
const extendHex = shortHex =>
  '#' + shortHex.slice(shortHex.startsWith('#') ? 1 : 0).split('').map(x => x + x).join('');
// extendHex('#03f') -> '#0033ff'
// extendHex('05a') -> '#0055aa'
```