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/DnsMasq.php

@@ -51,6 +51,23 @@ function install($tld = 'test')
         info('Valet is configured to serve for TLD [.'.$tld.']');
     }
 
+    /**
+     * Forcefully uninstall dnsmasq.
+     * 
+     * @return void
+     */
+    function uninstall()
+    {
+        $this->brew->stopService('dnsmasq');
+        $this->brew->uninstallFormula('dnsmasq');
+        $this->cli->run('rm -rf /usr/local/etc/dnsmasq.d/dnsmasq-valet.conf');
+    }
+
+    /**
+     * Tell Homebrew to restart dnsmasq
+     * 
+     * @return void
+     */
     function restart()
     {
         $this->brew->restartService('dnsmasq');