Use Werror on CI

postgrest.cabal

@@ -12,6 +12,14 @@ maintainer:            cred+github@begriffs.com
 category:              Web
 build-type:            Simple
 cabal-version:         >=1.10
+source-repository head
+  type: git
+  location: git://github.com/begriffs/postgrest.git
+
+Flag CI
+  Description: No warnings allowed in continuous integration
+  Manual:      True
+  Default:     False
 
 executable postgrest
   main-is:             PostgREST/Main.hs
@@ -47,7 +55,11 @@ executable postgrest
   hs-source-dirs:      src
 
 library
-  ghc-options:         -Wall -W -O2
+  if flag(ci)
+    ghc-options:       -Wall -W -Werror
+  else
+    ghc-options:       -Wall -W -O2
+
   default-language:    Haskell2010
   default-extensions:  OverloadedStrings, ScopedTypeVariables, QuasiQuotes
   build-depends:       base >=4.6 && <5
@@ -91,7 +103,10 @@ Test-Suite spec
   Default-Language:    Haskell2010
   default-extensions:  OverloadedStrings, ScopedTypeVariables, QuasiQuotes
   Hs-Source-Dirs:      test, src
-  ghc-options:         -Wall -W -Werror
+  if flag(ci)
+    ghc-options:       -Wall -W -Werror
+  else
+    ghc-options:       -Wall -W -O2
   Main-Is:             Main.hs
   Other-Modules:       PostgREST.App
                      , PostgREST.Auth