install cabal-install in "global" as opposed to "user" mode

Avoids initializing anything in the home directory
add :haskell dependency type that checks ghc-pkg

When detecting a missing package, it suggests that you install it with:

  cabal --install global <PACKAGE>

Example usage in formulae:

  depends_on 'parsec' => :haskell
add :cabal special dependency

Allows formulae to conditionally depend on "cabal-install" if `cabal`
executable is not present. Otherwise, `cabal` might have already been
provided from "haskell-platform" or by manual install.

Usage:

  depends_on :cabal
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.