bool support for user editable parameters

Hey, first thanks for this awesome project!
I found it comfortable to have checkboxes as parameters around and so I added them :)
I wasn't 100% sure how to handle the paramdef.default value… I implemented it so that you have to specify the value in the .jscad file as a boolean value - if you write p.e. `default = "true"` it throws an error.

Cheers, Jan