Further indent HTML attribute values

[JaapJoris]

This PR implements option 3 from issue #53, because I think that option is the most consistent. There remain a few edge cases that I did not account for, such as:

<path d="
    M 80 80
    A 45 45, 0, 0, 0, 125 125
    L 125 80 Z
"
    fill="green"
/>

Closes #53