add shellcheck, a static analysis tool for shell scripts

The project doesn't seem to have a version, so this is dubbed v0.0.1 and is frozen to the git SHA where the current master branch is.

Two problems with this formula's build process:

- `cabal update` - Necessary to run at least once after Cabal has been installed to fetch the list of available packages. However, this writes the cached files to `~/.cabal` (i.e. outside of Homebrew's prefix).
- `cabal install ...` - Also writes some files to `~/.cabal`, even with `--global` flag enabled. This doesn't perform the checks whether the packages are already installed. Is it worth adding a Haskell package dependency checker to Homebrew for this?