farver::decode_colour() could accept 3 character hex codes

3 character codes like `#9aa` are a useful shorthand, and are valid in HTML/CSS.
Can farver be edited so as to accept these (and presumably convert them to the full 6 character code?)

```r
farver::decode_colour("#9aa")
✘ Error: Malformed colour string `#9aa`. Must contain either 6 or 8 hex values
```