Replace postinstall script with prepare

[djcsdy]

It is not safe to run pinst in the prepack script.

See also #24, #23, #22.

It seems like a bad idea to keep the broken prepack script in the source of pinst itself.

Since we build and publish pinst using npm, not yarn, we can run husky in the prepare step instead of postinstall. Then we don't need to use pinst to publish pinst :-).