Some hlint fixes.

Ironically, this is the first time the linter has been linted.
Improved $(echo ..) warnings
Warn about using 'su foo' to continue as foo
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.
Allow \n before and after ||/&& in [[ ]]
Merge pull request #105 from Dridi/dynamic_link

Dynamic linking to libShellCheck
Added support for |&
Merge branch 'master' of github.com:koalaman/shellcheck
Fixed x[0] not recognized as reference of x in arithmetics
Don't warn about decimals when comparing with =
Warn about functions using parameters that are never passed
Warn about array assignments for /bin/sh
Don't warn about commas when quoted in a=("a,b")
Check attempts to set variables with 'set'