Rework --shell option into --generate-hook to avoid option conflict

The Symfony Framework console has a global --shell option which makes 0.4.0
of this module incompatible with it. Conflicts can't be avoided with everything,
but since use with Symphony Framework is one of the main use cases for this
module, it's worth avoiding a conflict.

Resolves #6

src/CompletionCommand.php

@@ -33,20 +33,14 @@ protected function configure()
             ->addOption(
                 'generate-hook',
                 'g',
-                InputOption::VALUE_NONE,
+                InputOption::VALUE_OPTIONAL,
                 'Generate BASH code that sets up completion for this application.'
             )
             ->addOption(
                 'program',
                 'p',
                 InputOption::VALUE_REQUIRED,
                 "Program name that should trigger completion\n<comment>(defaults to the absolute application path)</comment>."
-            )
-            ->addOption(
-                'shell',
-                's',
-                InputOption::VALUE_REQUIRED,
-                "Force the shell to generate a hook for <comment>(" . implode(', ', HookFactory::getShellTypes()) . ")</comment>\n"
             );
     }
 
@@ -61,7 +55,7 @@ protected function execute(InputInterface $input, OutputInterface $output)
 
             $factory = new HookFactory();
             $hook = $factory->generateHook(
-                $input->getOption('shell') ?: $this->getShellType(),
+                $input->getOption('generate-hook') ?: $this->getShellType(),
                 $program,
                 $input->getOption('program')
             );
@@ -85,7 +79,7 @@ protected function runCompletion()
     protected function getShellType()
     {
         if (!getenv('SHELL')) {
-            throw new \RuntimeException('Could not read SHELL environment variable. Please specify your shell type with the --shell option');
+            throw new \RuntimeException('Could not read SHELL environment variable. Please specify your shell type as an argument to the --generate-hook option.');
         }
 
         return basename(realpath(getenv('SHELL')));