BohemianCoding/Sketch#3319 Script import: Add support for absolute and ~ paths

I got confused with branch names, so I deleted the old one and created a named feature branch instead.

My comments still stand on the previous PR though:
https://github.com/BohemianCoding/CocoaScript/pull/1#discussion_r21744115

Ref BohemianCoding/Sketch#3319