Add optional wrist rest to magic trackpad ergonomic support

[jyaganeh]

This changes the magic_trackpad_ergonomic_support.scad file to include an optional wrist rest.

The wrist_rest parameter can be used to enable/disable the wrist rest platform. When set to no, the original model is produced.
The wrist_rest_length parameter specifies the length of the wrist rest in centimeters.

If you can think of a cleaner way to case on the value of wrist_rest, I'd be happy to update this PR.

[ex-nerd]

I apparently had notifications disabled and never saw this come in. I've since made some significant changes to the design, so I think I'll implement this on my own unless you're up for fixing the merge conflicts.

[jyaganeh]

I wanted to print another one of these, so I went ahead and fixed the merge conflicts. Feel free to merge or just close this PR if you'd rather implement this yourself.