Reformat ShellCheck.cabal for readability

Uses one line per `build-depends' or `exposed-modules'. Also got them
sorted by name. Folded `base' dependencies into a single one.
Make the executable depend on the library

It needed a bump to 1.8 for the minimum `Cabal-Version'. One downside is
that the executable also build-depends on the same libraries.
Alphabetical order is kept, except for the dependency to the ShellCheck
library itself.
Merge pull request #105 from Dridi/dynamic_link

Dynamic linking to libShellCheck