Respect prefers-reduced-motion setting

[sophiekoonin]

Extremely copy-pastey, but it seems like each cursor is its own encapsulated thing so it seemed like the thing to do.

• check prefers-reduced-motion when we init the cursor trail
• when the value of prefers-reduced-motion changes, either re-init or destroy the cursor trail
• add an id to the trail's canvas element, so that we can remove it from the DOM where necessary (this may help with #33?)

[sophiekoonin]

ah crap I seem to be working off an old version, gimme a sec