Add commandline flag --no-user-config

Stops brittany from trying to read a user-global config
flag. Together with --config-file, this can be used to pass
one single config to brittany, thereby controlling exactly
and explicit what the inputs of brittany are.
Should be useful for testing stuff that might depend on
config.

src-brittany/Main.hs

@@ -146,6 +146,7 @@ mainCmdParser helpDesc = do
   printHelp    <- addSimpleBoolFlag "h" ["help"] mempty
   printVersion <- addSimpleBoolFlag "" ["version"] mempty
   printLicense <- addSimpleBoolFlag "" ["license"] mempty
+  noUserConfig <- addSimpleBoolFlag "" ["no-user-config"] mempty
   configPaths  <- addFlagStringParams ""
                                       ["config-file"]
                                       "PATH"
@@ -217,7 +218,11 @@ mainCmdParser helpDesc = do
       else pure configPaths
 
     config <-
-      runMaybeT (readConfigsWithUserConfig cmdlineConfig configsToLoad)
+      runMaybeT
+          (if noUserConfig
+            then readConfigs cmdlineConfig configsToLoad
+            else readConfigsWithUserConfig cmdlineConfig configsToLoad
+          )
         >>= \case
               Nothing -> System.Exit.exitWith (System.Exit.ExitFailure 53)
               Just x  -> return x