Filter --keep-public-path from arguments (fixes #180)

The webpack-dev-server dose not know the --keep-public-path argument
and fails with an 'Unknown Argument' error if it is aplied.

So before webpack-encore hands over controle to webpack-cli/-dev-server it has to
remove arguments that only exist for webpack-encore.

Author: Andreas Freimuth

bin/encore.js

@@ -23,6 +23,10 @@ context.runtimeConfig = runtimeConfig;
 // remove the command from the output
 process.argv.splice(2, 1);
 
+// remove arguments not supported by webpack/-dev-server
+const EncoreOnlyArguments = new Set(['--keep-public-path']);
+process.argv = process.argv.filter(arg => !EncoreOnlyArguments.has(arg));
+
 if (!runtimeConfig.isValidCommand) {
     if (runtimeConfig.command) {
         console.log(chalk.bgRed.white(`Invalid command "${runtimeConfig.command}"`));