Move away from .sh scripts, replace grunt-shell w/ grunt-exec

The .sh scripts haven't been popular with maintainers, who seem to be allergic to them, as if npm installing something doesn't run a higher risk of that npm package deleting your files, given that it's far less scrutable than a `.sh` script. Oh well.

Also replaced grunt-shell with grunt-exec because of [rudeness against a README improvement from the author of grunt-shell](https://github.com/sindresorhus/grunt-shell/pull/85).