fix warning

the new version is slightly more point-free.

the warning was:
  src/System/Console/Hawk/Config.hs:183:14: Warning:
      This binding for `exts' shadows the existing binding
        bound at src/System/Console/Hawk/Config.hs:182:35
fix warning

the new version will print a slightly more comprehensible, automated
error message instead of just "undefined".

the warning was:
  src/System/Console/Hawk/Config.hs:200:13: Warning:
      Pattern match(es) are overlapped
      In a case alternative: _ -> ...
fix warnings

by cleaning up unused imports.

the warnings were:
  src/System/Console/Hawk/Options.hs:5:1: Warning:
      The import of `Control.Arrow' is redundant
        except perhaps to import instances from `Control.Arrow'
      To import instances alone, use: import Control.Arrow()

  src/System/Console/Hawk/Options.hs:11:1: Warning:
      The import of `Data.Maybe' is redundant
        except perhaps to import instances from `Data.Maybe'
      To import instances alone, use: import Data.Maybe()