Allow `_completion` command to auto-complete it's arguments or their values

[aik099]

Right now the CompletionCommand has no arguments, but there might be added in #30. In that case without this PR the auto-complete for app _completion argument won't work.

In unit tests we're testing all parts separately, but we don't test by really invoking completion hook. That's why I don't know how do I test the changes I've made. All existing tests still pass.

Closes #46

[stecman]

Nice spotting. This might be achievable with less code:

diff --git a/src/CompletionHandler.php b/src/CompletionHandler.php
index 0925df0..cce8fb4 100644
--- a/src/CompletionHandler.php
+++ b/src/CompletionHandler.php
@@ -103,6 +103,10 @@ class CompletionHandler

         try {
             $this->command = $this->application->find($cmdName);
+
+            // Create a new instance of the command to avoid any changes made by Command::run
+            $commandClass = get_class($this->command);
+            $this->command = new $commandClass();
         } catch (\InvalidArgumentException $e) {
             // Exception thrown, when multiple or none commands are found.
         }

I guess that doesn't cover cases where the constructor of a command has been changed though..

[aik099]

That might work, but the application won't be available from the command and we need to do ->setApplication call too. And then it would look like code duplication. So using getNativeDefinition is recommended way. I'll make createDefinition method protected (it's private now because I copy-pased it from ListCommand) and people then can change it to alter command definition.