Import: Add support for absolute and ~ paths

Import paths can be in the format:

    relative/to/script.js
    /absolute/path/to/script.js
    ~/relative/to/home/script.js
import: reinstate warning for empty base in non-absolute URLs
Merge pull request #2 from BohemianCoding/feature/3319

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