Allow Valet to uninstall itself

`valet uninstall` only displays information about how to manually uninstall and clean up after Valet.

This PR adds a `--force` parameter, which will forcefully remove Valet and the Homebrew services it installs, as well as clean up the config files and log files.
But for a few post-uninstall composer dependencies, cleanup is very thorough.

This brings idempotency to both `valet install` and `valet uninstall --force`

(There may still be edge cases where other Homebrew or composer packages might create interference with install/uninstall, but this makes things much easier to self-troubleshoot.)

cli/Valet/PhpFpm.php

@@ -45,6 +45,18 @@ function install()
         $this->restart();
     }
 
+    /**
+     * Forcefully uninstall all of Valet's supported PHP versions and configurations
+     * 
+     * @return void
+     */
+    function uninstall()
+    {
+        $this->brew->uninstallAllPhpVersions();
+        rename('/usr/local/etc/php', '/usr/local/etc/php-valet-bak'.time());
+        $this->cli->run('rm -rf /usr/local/var/log/php-fpm.log');
+    }
+
     /**
      * Update the PHP FPM configuration.
      *