Add optional css indentation for humans: grad.py

[a-x-]

--human-readable

produces something like this:

.gradify-0 {
    background: rgb(249,244,238);
    background:
        -webkit-linear-gradient(180deg, rgba(249,244,238,1) 0%, rgba(249,244,238,0) 100%),
        -webkit-linear-gradient(0deg, rgba(21,80,148,1) 0%, rgba(21,80,148,0) 100%),
        -webkit-linear-gradient(90deg, rgba(121,128,209,1) 0%, rgba(121,128,209,0) 100%),
        -webkit-linear-gradient(270deg, rgba(206,113,47,1) 0%, rgba(206,113,47,0) 100%);
        linear-gradient(180deg, rgba(249,244,238,1) 0%, rgba(249,244,238,0) 100%),
        linear-gradient(0deg, rgba(21,80,148,1) 0%, rgba(21,80,148,0) 100%),
        linear-gradient(90deg, rgba(121,128,209,1) 0%, rgba(121,128,209,0) 100%),
        linear-gradient(270deg, rgba(206,113,47,1) 0%, rgba(206,113,47,0) 100%),
}